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Reaction-based machine learning representations for predicting the enantioselectivity of organocatalysts


Simone Gallarati, Raimon Fabregat, Rubén Laplaza, Sinjini Bhattacharjee, Matthew Wodrich, Clemence Corminboeuf

  • Hundreds of catalytic methods are developed each year to meet the demand for high-purity chiral compounds. The computational design of enantioselective organocatalysts remains a significant challenge, as catalysts are typically discovered through experimental screening. Recent advances in combining quantum chemical computations and machine learning (ML) hold great potential to propel the next leap forward in asymmetric catalysis. Within the context of quantum chemical machine learning (QML, or atomistic ML), the ML representations used to encode the structure of molecules and evaluate their similarity cannot easily capture the subtle energy differences that govern enantioselectivity. Here, we present a general strategy for improving molecular representations within an atomistic machine learning model to predict the enantiomeric excess of asymmetric propargylation organocatalysts solely from the structure of catalytic cycle intermediates. Mean absolute errors as low as 0.25 kcal ...

Latest version: v1
Publication date: Mar 05, 2021

Importance of surface oxygen vacancies for ultrafast hot carrier relaxation and transport in Cu2O


Chiara Ricca, Ulrich Aschauer, Lisa Grad, Matthias Hengsberger, Jürg Osterwalder

  • Cu2O has appealing properties as an electrode for photo-electrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies (VO) in the experimentally observed (√3×√3)R30° reconstruction of the dominant (111) surface. Our results show that these VO are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged VO plays a crucial role in the non-radiative electron capture process with a capture coefficient of about 10^-9 cm3/s and a lifetime of 0.04 ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface VO chemistry will be a crucial step in optimizing Cu2O for photoelectrode applications.

Latest version: v1
Publication date: Mar 05, 2021

Asymmetric azide‐alkyne Huisgen cycloaddition on chiral metal surfaces


Samuel Stolz, Michael Bauer, Carlo A. Pignedoli, Nils Krane, Max Bommert, Elia Turco, Nicolo Bassi, Amogh Kinikar, Néstor Merino-Dìez, Roland Hany, Harald Brune, Oliver Gröning, Roland Widmer

  • The record contains the data supporting our recent findings on asymmetric azide-alkyne Huisgen cycloaddition on chiral metal surfaces: Achieving fundamental understanding of enantioselective heterogeneous synthesis is marred by the permanent presence of multitudinous arrangements of catalytically active sites in real catalysts. We address this issue by using structurally comparatively simple, well‐defined, and chiral intermetallic PdGa{111} surfaces as catalytic substrates. We demonstrate the impact of chirality transfer and ensemble effect for the thermally activated azide‐alkyne Huisgen cycloaddition between 3‐(4‐azidophenyl)propionic acid and 9‐ethynylphenanthrene on these threefold symmetric intermetallic surfaces under ultrahigh vacuum conditions. Specifically, we encounter a dominating ensemble effect for this reaction as on the Pd3‐terminated PdGa{111} surfaces no stable heterocoupled structures are created, while on the Pd1‐terminated PdGa{111} surfaces, the cycloaddition ...

Latest version: v1
Publication date: Mar 02, 2021

Reversible dehalogenation in on-surface aryl-aryl coupling


Samuel Stolz, Marco Di Giovannantonio, José I. Urgel, Qiang Sun, Amogh Kinikar, Gabriela Borin Barin, Max Bommert, Roman Fasel, Roland Widmer

  • The record contains the data to support the findings of our recent work on reversibility of the dehalogenation process in on-surface aryl-aryl coupling. In the emerging field of on‐surface synthesis, dehalogenative aryl–aryl coupling is unarguably the most prominent tool for the fabrication of covalently bonded carbon‐based nanomaterials. Despite its importance, the reaction kinetics are still poorly understood. Here we present a comprehensive temperature‐programmed x‐ray photoelectron spectroscopy investigation of reaction kinetics and energetics in the prototypical on‐surface dehalogenative polymerization of 4,4′′‐dibromo‐p‐terphenyl into poly(para‐phenylene) on two coinage metal surfaces, Cu(111) and Au(111). We find clear evidence for reversible dehalogenation on Au(111), which is inhibited on Cu(111) owing to the formation of organometallic intermediates. The incorporation of reversible dehalogenation in the reaction rate equations leads to excellent agreement with ...

Latest version: v1
Publication date: Mar 02, 2021

Extensive benchmarking of DFT+U calculations for predicting band gaps


Nicole Kirchner-Hall, Wayne Zhao, Yihuang Xiong, Iurii Timrov, Ismaila Dabo

  • Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is ...

Latest version: v1
Publication date: Mar 02, 2021

Building a consistent and reproducible database for adsorption evaluation in Covalent-Organic Frameworks


Daniele Ongari, Aliaksandr V. Yakutovich, Leopold Talirz, Berend Smit

  • We present a workflow that traces the path from the bulk structure of a crystalline material to assessing its performance in carbon capture from coal’s postcombustion flue gases. This workflow is applied to a database of 324 covalent−organic frameworks (COFs) reported in the literature, to characterize their CO2 adsorption properties using the following steps: (1) optimization of the crystal structure (atomic positions and unit cell) using density functional theory, (2) fitting atomic point charges based on the electron density, (3) characterizing the pore geometry of the structures before and after optimization, (4) computing carbon dioxide and nitrogen isotherms using grand canonical Monte Carlo simulations with an empirical interaction potential, and finally, (5) assessing the CO2 parasitic energy via process modeling. The full workflow has been encoded in the Automated Interactive Infrastructure and Database for Computational Science (AiiDA). Both the workflow and the ...

Latest version: v8
Publication date: Feb 24, 2021

Bias free multiobjective active learning for materials design and discovery


Kevin Maik Jablonka, Giriprasad Melpatti Jothiappan, Shefang Wang, Berend Smit, Brian Yoo

  • The design rules for materials are clear for applications with a single objective. For most applications, however, there are often multiple, sometimes competing objectives where there is no single best material, and the design rules change to finding the set of Pareto optimal materials. In this work, we introduce an active learning algorithm that directly uses the Pareto dominance relation to compute the set of Pareto optimal materials with desirable accuracy. We apply our algorithm to de novo polymer design with a prohibitively large search space. Using molecular simulations, we compute key descriptors for dispersant applications and reduce the number of materials that need to be evaluated to reconstruct the Pareto front with a desired confidence by over 98% compared to random search. This work showcases how simulation and machine learning techniques can be coupled to discover materials within a design space that would be intractable using conventional screening approaches.

Latest version: v1
Publication date: Feb 22, 2021

Effect of density, phonon scattering and nanoporosity on the thermal conductivity of anisotropic cellulose nanocrystal foams


Varvara Apostolopoulou-Kalkavoura, Pierre Munier, Lukasz Dlugozima, Veit-Lorenz Heuthe, Lennart Bergström

  • Ice templated anisotropic foams based on cellulose nanocrystals (CNC) with densities ranging between 25 to 129 kg.m-3 were prepared from aqueous CNC dispersions. The thermal conductivities perpendicular to the columnar macropores direction increased in a non-monotonous way with the increasing CNC foam density while the thermal conductivity reached a minimum value (24 mW m-1 K-1 at 20% RH and 295 K) for the CNC foam with the highest nanoporosity. Summation of the theoretical thermal conductivity calculations of the solid and gas conduction within the foams as well as the thermal conductivity of water showed that phonon scattering at the solid-solid interfaces is responsible for reaching very low thermal conductivity values. The foam wall nanoporosity, the particle alignment, the macropores orientation and the foam wall thickness seem to have a minimal effect on the thermal conductivity but can explain the deviations between the theoretical estimates and the experimental data. To ...

Latest version: v1
Publication date: Feb 16, 2021

Machine learning for metallurgy: a neural network potential for Al-Mg-Si


Abhinav C. P. Jain, Daniel Marchand, Albert Glensk, Michele Ceriotti, W. A. Curtin

  • High-strength metal alloys achieve their performance via careful control of the nucleation, growth, and kinetics of precipitation. Alloy mechanical properties are then controlled by atomic scale phenomena such as shearing of the precipitates by dislocations. Atomistic modeling to understand the operative mechanisms requires length and time scales far larger than those accessible by first-principles methods. Here, a family of Behler-Parinello neural-network potentials (NNPs) for the Al-Mg-Si system is developed to enable quantitative studies of Al-6xxx alloys. The NNP is trained on metallurgically-important quantities computed by first principles density functional theory (DFT) leading to high fidelity predictions of intermetallic compounds, elastic constants, dilute solid-solution energetics, precipitate/matrix interfaces, Al stacking fault energies, antisite defect energies, and other quantities. A preliminary examination of early-stage clustering kinetics and energetics in ...

Latest version: v1
Publication date: Feb 09, 2021

Appraisal of calcium ferrites as cathodes for calcium rechargeable batteries: DFT, synthesis, characterization and electrochemistry of Ca4Fe9O17


M. Elena Arroyo-de Dompablo, José Luis Casals

  • Sustainability combined with high energy density prospects makes Fe-based oxides attractive as cathodes for calcium rechargeable batteries. This work presents a DFT evaluation of the CaFe2+nO4+n (0 < n < 3) family, for which both the average intercalation voltage and the theoretical specific capacity decrease with the increasing n value. The term n = 1/4, Ca4Fe9O17, meets the most appealing characteristics: a calculated average voltage of 4.16 V, a theoretical specific capacity of 230 mA h g−1 and the lowest energy barrier for Ca migration so far predicted for an existing oxide (0.72 eV). To overcome the previously reported synthesis difficulties, we employed a novel synthesis procedure in sealed quartz tubes followed by quenching in water. The XRD and SAED patterns of the prepared Ca4Fe9O17 powder reveal a certain degree of stacking defects along the c axis. Attempts to deinsert Ca ions from Ca4Fe9O17 by chemical means (NO2BF4 in ACN) and in electrochemical Ca cells were ...

Latest version: v1
Publication date: Feb 07, 2021

Efficient Kr/Xe separation from triangular g-C3N4 nanopores: density-functional theory calculations benchmarked with random phase approximation


Mohammad Tohidivahdat, Davide Campi, Nicola Colonna, Luis Francisco Villalobos, Nicola Marzari, Kumar Agrawal Varoon

  • Poly(triazine imide) or PTI is a promising material for molecular sieving membranes, thanks to its atom-thick ordered lattice with an extremely high density (1.6 × 10^14 pores/cm2) of triangular-shaped nanopores of ~0.34 nm diameter. Here, we investigate the application of PTI nanopores in the purification of Kr from Xe to reduce the storage volume of the mixture of 85Kr/Xe. Using van-der-Waals density-functional theory (vdW-DFT) calculations, benchmarked against the random phase approximation (RPA), we calculate the potential energy profiles for Kr and Xe across the nanopores. For each gas, starting from the RPA potential-energy profile, the force-field parameters to be used in the classical molecular dynamics framework are trained to calculate the Helmholtz free energy barrier as a function of temperature, and therefore, the corresponding entropic loss. Overall, due to the much higher activation energy from the adsorbed state in Xe (17.61 and 42.10 kJ/mole for Kr and Xe, ...

Latest version: v1
Publication date: Feb 07, 2021

Biomimetic high performance artificial muscle built on sacrificial coordination network and mechanical training process


Zhikai Tu, Weifeng Liu, Jin Wang, Jinhao Huang, Jinxing Li, Hongming Lou, Xueqing Qiu

  • Artificial muscle materials promise incredible applications in actuators, robotics and medical apparatus, yet the ability to mimic the full characteristics of skeletal muscles into synthetic materials remains a huge challenge. Herein, inspired by the dynamic sacrificial bonds in biomaterials and the self-strengthening of skeletal muscles by physical exercise, high performance artificial muscle material is prepared by rearrangement of sacrificial coordination bonds in the polyolefin elastomer via a repetitive mechanical training process. Biomass lignin is incorporated as a green reinforcer for the construction of interfacial coordination bonds. The prepared artificial muscle material exhibits high actuation strain (>40%), high actuation stress (1.5 MPa) which can lift more than 10000 times its own weight with 30% strain, characteristics of excellent self-strengthening by mechanical training, strain-adaptive stiffening, and heat/electric programmable actuation performance. In this ...

Latest version: v1
Publication date: Feb 07, 2021

DFT investigation of Ca mobility in reduced-perovskite and oxidized-marokite oxides


M. Elena Arroyo-de Dompablo, José Luis Casals

  • Progress in the development of rechargeable Ca-ion batteries demands the discovery of potential cathode materials. Transition metal oxides are interesting candidates due to their theoretical high energy densities, but with the drawback of a low Ca mobility. Previous computational/experimental investigations associate the electrochemical inactivity of various oxides (CaMO3-perovskite, CaMn2O4-post-spinel and CaV2O5) to high energy barriers for Ca migration. The introduction of oxygen and/or Ca vacancies in ternary transition metal oxides is a likely way to reshape the local topology and hence improve the Ca diffusivity. In this work, the energy barriers for Ca migration are calculated and discussed for (i) oxygen-deficient perovskites within the related Ca2Fe2O5-brownmillerite and Ca2Mn2O5 structures, and (ii) tunnel CaMn4O8, a derivative of the CaMn2O4-marokite with Ca vacancies.

Latest version: v1
Publication date: Feb 07, 2021

Radial spin texture of the Weyl fermions in chiral tellurium


G. Gatti, D. Gosálbez-Martínez, S. S. Tsirkin, M. Fanciulli, M. Puppin, S. Polishchuk, S. Moser, L. Testa, E. Martino, S. Roth, Ph. Bugnon, L. Moreschini, A. Bostwick, C. Jozwiak, E. Rotenberg, G. Di Santo, L. Petaccia, I. Vobornik, J. Fujii, J. Wong, D. Jariwala, H. A. Atwater, H. M. Rønnow, M. Chergui, O. V. Yazyev, M. Grioni, A. Crepaldi

  • In the present record we provide the theoretical calculations used in the article: G. Gatti et al., Radial Spin Texture of the Weyl Fermions in Chiral Tellurium, Phys. Rev. Lett. 125, 216402. It consist of a detailed analysis of the electronic structure and spin textures at different points of the Brillouin zone. We provide the band structure and spin expectation values along different high-symmetry lines, Fermi surface contour plots in the ΓMLA plane, and spin texture in small sphere around two points of the Brillouin zone. The data is provided as the output format of the Quantum Espresso package. We also include the pseudopotential used in these calculations.

Latest version: v1
Publication date: Jan 27, 2021

Structural involvement in the melting of the charge density wave in 1T-TiSe2


Max Burian, Michael Porer, Jose R.L. Mardegan, Vincent Esposito, Sergii Parchenko, Burganov Bulat, Namrata Gurung, Mahesh Ramakrishnan, Valerio Scagnoli, Hirkoi Ueda, Sonia Francoual, Federica Fabrizi, Yoshikazu Tanaka, Tadashi Togashi, Kai Rossnagel, Steven L. Johnson, Urs Staub

  • The simultaneous condensation of electronic and structural degrees of freedom gives rise to new states of matter, including superconductivity and charge-density-wave formation. When exciting such a condensed system, it is commonly assumed that the ultrafast laser pulse disturbs primarily the electronic order, which in turn destabilizes the atomic structure. Contrary to this conception, we show here that structural destabilization of few atoms causes macroscopic melting of the charge-density wave in 1T-TiSe2. In detail, we use ultrafast pump-probe non-resonant and resonant X-ray diffraction to track the periodic lattice distortion and the electronic charge density wave in 1T-TiSe2 upon optical excitation. We observe a fluence regime in which the periodic lattice deformation is strongly suppressed but the charge density wave related Se 4p orbital order remains mostly intact. Complete melting of both structural and electronic order occurs 4-5 times faster than expected from a purely ...

Latest version: v1
Publication date: Jan 27, 2021

Screening from eg states and antiferromagnetic correlations in d(1,2,3) perovskites: A GW+EDMFT investigation


Francesco Petocchi, Fredrik Nilsson, Ferdi Aryasetiawan, Philipp Werner

  • We perform a systematic ab initio study of the electronic structure of Sr(V,Mo,Mn)O3 perovskites, using the parameter-free GW+EDMFT method. This approach self-consistently calculates effective interaction parameters, taking into account screening effects due to nonlocal charge fluctuations. Comparing the results of a 3-band (t2g) description to those of a 5-band (t2g+eg) model, it is shown that the eg states have little effect on the low-energy properties and the plasmonic features for the first two compounds but play a more active role in SrMnO3. In the case of SrMnO3 paramagnetic GW+EDMFT yields a metallic low-temperature solution on the verge of a Mott transition, while antiferromagnetic GW+EDMFT produces an insulating solution with the correct gap size. We discuss the possible implications of this result for the nature of the insulating state above the Neel temperature, and the reliability of the GW+EDMFT scheme.

Latest version: v1
Publication date: Jan 27, 2021

Normal State of Nd_(1-x)Sr_xNiO2 from Self-Consistent GW+EDMFT


Francesco Petocchi, Viktor Christiansson, Fredrik Nilsson, Ferdi Aryasetiawan, Philipp Werner

  • Superconductivity with a remarkably high Tc has recently been observed in hole-doped NdNiO2, a material that shares similarities with the high-Tc cuprates. This discovery promises new insights into the mechanism of unconventional superconductivity, but at the modeling level, there are fundamental issues that need to be resolved. While it is generally agreed that the low-energy properties of cuprates can, to a large extent, be captured by a single-band model, there has been a controversy in the recent literature about the importance of a multiband description of the nickelates. Here, we use a multisite extension of the recently developed GW+EDMFT method, which is free of adjustable parameters, to self-consistently compute the interaction parameters and electronic structure of hole-doped NdNiO2. This full ab initio simulation demonstrates the importance of a multiorbital description, even for the undoped compound, and it produces results for the resistivity and Hall conductance in qualitative agreement with experiment.

Latest version: v1
Publication date: Jan 27, 2021

Torsional stress can regulate the unwrapping of two outer half superhelical turns of nucleosomal DNA


Hisashi Ishida, Hidetoshi Kono

  • Torsional stress has a significant impact on the structure and stability of the nucleosome. RNA polymerase imposes torsional stress on the DNA in chromatin and unwraps the DNA from the nucleosome to access the genetic information encoded in the DNA. To understand how the torsional stress affects the stability of the nucleosome, we examined the unwrapping of two half superhelical turns of nucleosomal DNA from either end of the DNA under torsional stress with all-atom molecular dynamics simulations. The free energies for unwrapping the DNA indicate that positive stress that overtwists DNA facilitates a large-scale asymmetric unwrapping of the DNA without a large extension of the DNA. During the unwrapping, one end of the DNA was dissociated from H3 and H2A-H2B while the other end of the DNA stably remained wrapped. The detailed analysis indicates that this asymmetric dissociation is facilitated by the geometry and bendability of the DNA under positive stress. The geometry ...

Latest version: v1
Publication date: Jan 27, 2021

Sampling enhancement by metadynamics driven by machine learning and de novo protein modelling


Kateřina Tomášková, Dalibor Trapl, Vojtěch Spiwok

  • Folding of villin miniprotein was studied by parallel tempering metadynamics driven by machine learning. To obtain a training set for machine learning, we generated a large series of structures of the protein by the de novo protein structure prediction package Rosetta. A neural network was trained to approximate the Rosetta score. Parallel tempering metadynamics driven by this approximated Rosetta score successfully predicted the native structure and the free energy surface of the studied system. These files make it possible to rerun all simulations. The directory METAD contains input files for metadynamics (no folding events observed). The directory PT-METAD contains input files for parallel tempering metadynamics. All simulations were done using Gromacs 2016.4, Anncolvar 0.8, Plumed 2.4 and OpenMPI 4.0.0.

Latest version: v3
Publication date: Jan 26, 2021

Thermodynamics of order and randomness in dopant distributions inferred from atomically resolved images


Lukas Vlcek, Shize Yang, Yongji Gong, Pulickel Ajayan, Wu Zhou, Matthew Chisholm, Maxim Ziatdinov, Rama Vasudevan, Sergei Kalinin

  • Exploration of structure-property relationships as a function of dopant concentration is commonly based on mean field theories for solid solutions. However, such theories that work well for semiconductors tend to fail in materials with strong correlations, either in electronic behavior or chemical segregation. In these cases, the details of atomic arrangements are generally not explored and analyzed. The knowledge of the generative physics and chemistry of the material can obviate this problem, since defect configuration libraries as stochastic representation of atomic level structures can be generated, or parameters of mesoscopic thermodynamic models can be derived. To obtain such information for improved predictions, we use data from atomically resolved microscopic images that visualize complex structural correlations within the system and translate them into statistical mechanical models of structure formation. Given the significant uncertainties about the microscopic aspects ...

Latest version: v1
Publication date: Jan 26, 2021

Confinement effects and acid strength in Zeolites


Emanuele Grifoni, GiovanniMaria Piccini, Johannes Lercher, Vassiliki-Alexandra Glezakou, Roger Rousseau, Michele Parrinello

  • Chemical reactivity and sorption in zeolites are coupled to confinement and - to a lesser extent- to the acid strength of Brønsted acid sites (BAS). In presence of water the zeolite Brønsted acid sites eventually convert into hydronium ions. The gradual transition from zeolite Brønsted acid sites to hydronium ions conversion in zeolites of varying pore size is examined by ab initio molecular dynamics combined with enhanced sampling based on well-tempered metadynamics and a recently developed set of collective variables. While at low water content (1-2 water/BAS) the acidic protons prefer to be shared between zeolites and water, higher water contents (n>2) invariably lead to solvation of the protons within a localized water cluster adjacent to the BAS. At low water loadings the standard free energy of the formed complexes is dominated by enthalpy and is associated with the acid strength of the BAS and the space around the site. Conversely, the entropy increases linearly with the ...

Latest version: v1
Publication date: Jan 26, 2021

Towards constant potential modeling of CO-CO coupling at liquid water-Cu(100) interfaces


Henrik H. Kristoffersen, Karen Chan

  • We have studied electrochemical *CO-*CO coupling in explicit electrolyte with density functional theory, molecular dynamics, and metadynamics. We considered both the *CO-*CO coupling reaction and the charging process required to keep the potential constant. The charging process consists of transferring explicit cations from the electrolyte and electrons from the potentiostat to the interface. Under constant charge conditions (non-constant electrostatic potential), the *CO-*CO coupling reaction energies are relative insensitive to the charge state at the interface and the electrolyte composition and the reaction occurs with co-adsorption of water. Under constant potential conditions, the *CO-*CO coupling reaction is stabilized at lower potentials because of charging and the reaction is influenced by the electrolyte composition. Here we have collected the data from the eight AIMD metadynamics simulations conducted in the study. Each AIMD data tar.gz file contains the VASP input ...

Latest version: v1
Publication date: Jan 26, 2021

Is a single conformer sufficient to describe the reorganization energy of amorphous organic transport materials?


J. Terence Blaskovits, Kun-Han Lin, Raimon Fabregat, Iwona Swiderska, Hélène Wu, Clémence Corminboeuf

  • The reorganization energy (λ), which quantifies the structural rearrangement of a molecule when accommodating a charge, is a key parameter in the evaluation of charge mobility in molecular solids. However, it is unclear how λ is influenced by conformational isomerism, which co-exist in amorphous solids. Here, we examine the conformational space of a family of model amorphous organic hole transport materials (HTMs), derived from triphenylamine in a core-arm template, and probe the effect of conformational complexity on λ. We observe an extreme dependence of λ on the conformer geometry of sterically congested HTMs, which to the best of our knowledge has not been described previously. These results serve as a cautionary tale that, while extracting the reorganization energy from a single molecular conformer optimized in the gas phase may be appropriate for rigid and sterically unencumbered structures, it is not for many state-of-the-art HTMs that contain multiple bulky substituents.

Latest version: v1
Publication date: Jan 25, 2021

High Li-ion conductivity in tetragonal LGPO: a comparative first-principles study against known LISICON and LGPS phases


Giulliana Materzanini, Leonid Kahle, Aris Marcolongo, Nicola Marzari

  • This work presents extensive first-principles (Car-Parrinello) molecular dynamics simulations of the solid-state electrolyte Li10GeP2O12 (LGPO) in a tetragonal phase -not synthesized so far- that is isostructural to the highly Li-ion conductive tetragonal phase of the sulfide analogue Li10GeP2S12 (LGPS). We provide comparative simulations of the experimentally known orthorhombic phase of LGPO (that we call here LISICON, from the family of superionic conductors to which LGPO belongs) and of the two experimentally known phases of LGPS, quasi-orthorhombic (called thio-LISICON) and tetragonal. We extract diffusion coefficients from fixed-cell simulations in the canonical ensemble and we study dynamical stability from variable-cell simulations in the isobaric-isothermal ensemble. The main outcome of this work is that, according to these simulations, although tetragonal LGPO is less stable than its orthorhombic allotrope, it exhibits a much higher conductivity, comparable to that ...

Latest version: v1
Publication date: Jan 22, 2021

Yield strength and misfit volumes of NiCoCr and implications for short-range-order


Binglun Yin, William Curtin

  • The face-centered cubic medium-entropy alloy NiCoCr has received considerable attention for its good mechanical properties, uncertain stacking fault energy, etc, some of which have been attributed to chemical short-range order (SRO). Here, we examine the yield strength and misfit volumes of NiCoCr to determine whether SRO has measurably influenced mechanical properties. Polycrystalline strengths show no systematic trend with different processing conditions. Measured misfit volumes in NiCoCr are consistent with those in random binaries. Yield strength prediction of a random NiCoCr alloy matches well with experiments. Finally, we show that standard spin-polarized density functional theory (DFT) calculations of misfit volumes are not accurate for NiCoCr. This implies that DFT may be inaccurate for other subtle structural quantities such as atom-atom bond distance so that caution is required in drawing conclusions about NiCoCr based on DFT. These findings all lead to the conclusion ...

Latest version: v1
Publication date: Jan 22, 2021

Mid-infrared radiative emission from bright hot plasmons in graphene


Laura Kim, Seyoon Kim, Pankaj Jha, Victor Brar, Harry Atwater

  • The decay dynamics of excited carriers in graphene have attracted wide scientific attention, as the gapless Dirac electronic band structure opens up relaxation channels that are not allowed in conventional materials. We report Fermi-level-dependent mid-infrared emission in graphene originating from a previously unobserved decay channel: hot plasmons generated from optically excited carriers. The observed Fermi-level dependence rules out Planckian light emission mechanisms and is consistent with the calculated plasmon emission spectra in photoinverted graphene. Evidence for bright hot plasmon emission is further supported by Fermi-level-dependent and polarization-dependent resonant emission from graphene plasmonic nanoribbon arrays under pulsed laser excitation. Spontaneous plasmon emission is a bright emission process as our calculations for our experimental conditions indicate that the spectral flux of spontaneously generated plasmons is several orders of magnitude higher than ...

Latest version: v1
Publication date: Jan 21, 2021

Smart local orbitals for efficient calculations within density functional theory and beyond


Guido Gandus, Angelo Valli, Daniele Passerone, Robert Stadler

  • The record contains data to support our research findings regarding the development of a novel method for deriving localized basis sets in the projector augmented wave formalism, allowing to obtain a reduced basis set of atomic orbitals through the subdiagonalization of each atomic block of the Hamiltonian. The resulting local orbitals (LOs) inherit the information of the local crystal field. In the LO basis, it becomes apparent that the Hamiltonian is nearly block-diagonal, and we demonstrate that it is possible to keep only a subset of relevant LOs that provide an accurate description of the physics around the Fermi level. This reduces to some extent the redundancy of the original basis set, and at the same time, it allows one to perform post-processing of DFT calculations, ranging from the interpretation of electron transport to extracting effective tight-binding Hamiltonians, very efficiently and without sacrificing the accuracy of the results.

Latest version: v1
Publication date: Jan 19, 2021

Analysis of minerals as electrode materials for Ca-based rechargeable batteries


M. Elena Arroyo-de Dompablo, Jose Luis Casals

  • Rechargeable lithium-ion batteries dominate the consumer electronics and electric vehicle markets. However, concerns on Li availability have prompted the development of alternative high energy density electrochemical energy storage systems. Rechargeable batteries based on a Ca metal anode can exhibit advantages in terms of energy density, safety and cost. The development of rechargeable Ca metal batteries requires the identification of suitable high specific energy cathode materials. This work focuses on Ca-bearing minerals because they represent stable and abundant compounds. Suitable minerals should contain a transition metal able of being reversibly reduced and oxidized, which points to several major classes of silicates and carbonates: olivine (CaFeSiO4; kirschsteinite), pyroxene (CaFe/MnSi2O6; hedenbergite and johannsenite, respectively), garnet (Ca3Fe/Cr2Si3O12; andradite and uvarovite, respectively), amphibole (Ca2Fe5Si8O22(OH)2; ferroactinolite) and double carbonates ...

Latest version: v1
Publication date: Jan 19, 2021

Magnetic exchange interactions in monolayer CrI₃ from many-body wavefunction calculations


Michele Pizzochero, Ravi Yadav, Oleg V. Yazyev

  • The marked interplay between the crystalline, electronic, and magnetic structure of atomically thin magnets has been regarded as the key feature for designing next-generation magneto-optoelectronic devices. In this respect, a detailed understanding of the microscopic interactions underlying the magnetic response of these crystals is of primary importance. Here, we combine model Hamiltonians with multireference configuration interaction wavefunctions to accurately determine the strength of the spin couplings in the prototypical single-layer magnet CrI₃. Our calculations identify the (ferromagnetic) Heisenberg exchange interaction J = −1.44 meV as the dominant term, being the inter-site magnetic anisotropies substantially weaker. We also find that single-layer CrI₃ features an out-of-plane easy axis ensuing from a single-ion anisotropy A = −0.10 meV, and predict g-tensor in-plane components gxx = gyy = 1.90 and out-of-plane component gzz  = 1.92. In addition, we assess the ...

Latest version: v1
Publication date: Jan 19, 2021

On-surface synthesis of singly and doubly porphyrin-capped graphene nanoribbon segments


Luis M. Mateo, Qiang Sun, Kristjan Eimre, Carlo A. Pignedoli, Tomas Torres, Roman Fasel, Giovanni Bottari

  • In this record we provide data to support our recent findings on the synthesis of porphyrin-capped graphene nanoribbons. On-surface synthesis has emerged as a powerful tool for the construction of large, planar, π-conjugated structures that are not accessible through standard solution chemistry. Among such solid-supported architectures, graphene nanoribbons (GNRs) hold a prime position for their implementation in nanoelectronics due to their manifold outstanding properties. Moreover, using appropriately designed molecular precursors, this approach allows the synthesis of functionalized GNRs, leading to nanostructured hybrids with superior physicochemical properties. Among the potential “partners” for GNRs, porphyrins (Pors) outstand due to their rich chemistry, robustness, and electronic richness, among others. However, the use of such π-conjugated macrocycles for the construction of GNR hybrids is challenging and examples are scarce. In a recent publication we report singly and ...

Latest version: v1
Publication date: Jan 19, 2021

Double-hybrid DFT functionals for the condensed phase: Gaussian and plane waves implementation and evaluation


Frederick Stein, Jürg Hutter, Vladimir V. Rybkin

  • Intermolecular interactions play an important role for the understanding of catalysis, biochemistry and pharmacy. Double-hybrid density functionals (DHDFs) combine the proper treatment of short-range interactions of common density functionals with the correct description of long-range interactions of wave-function correlation methods. Up to now, there are only a few benchmark studies available examining the performance of DHDFs in condensed phase. We studied the performance of a small but diverse selection of DHDFs implemented within Gaussian and plane waves formalism on cohesive energies of four representative dispersion interaction dominated crystal structures. We found that the PWRB95 and ωB97X-2 functionals provide an excellent description of long-ranged interactions in solids. In addition, we identified numerical issues due to the extreme grid dependence of the underlying density functional for PWRB95. The basis set superposition error (BSSE) and convergence with respect to ...

Latest version: v1
Publication date: Jan 19, 2021

Transport signatures of temperature-induced chemical potential shift and Lifshitz transition in layered type-II Weyl semimetal TaIrTe4


Yu Jian, QuanSheng Wu, Meng Yang, Qi Feng, Junxi Duan, Dongyun Chen, Qinsheng Wang, Wende Xiao, Youguo Shi, Oleg V. Yazyev, Yugui Yao

  • Temperature-induced Lifshitz transitions have been identified in several materials. Their chemical potential shows a substantial shift with changing temperature. The common feature of these materials is the coexistence of electron and hole pockets in the vicinity of the chemical potential. Here, we report the observation of temperature-induced chemical potential shift and Lifshitz transition in a layered type-II Weyl semimetal, TaIrTe4. The reversal of the polarity of the Hall resistivity and thermoelectric power (TEP) as the temperature increases clearly signal an appreciable shift of the chemical potential and change of the Fermi surface. It is corroborated by the improving agreement between the experimental TEP and the one calculated with temperature-dependent chemical potential. The complete disappearance of an electron pocket, consistent with the change of the Fermi surface when the chemical potential moves downwards, provides an evident signature of a temperature-induced Lifshitz transition in TaIrTe4.

Latest version: v1
Publication date: Jan 09, 2021

Large magnetoresistance and nonzero Berry phase in the nodal-line semimetal MoO2


Qin Chen, Zhefeng Hou, Shengnan Zhang, Binjie Xu, Yuxing Zhou, Huancheng Chen, Shuijin Chen, Jianhua Du, Hangdong Wang, Jinhu Yang, QuanSheng Wu, Oleg V. Yazyev, Minghu Fang

  • We performed calculations of the electronic band structure and the Fermi surface as well as measured the longitudinal resistivity ρxx(T,H), Hall resistivity ρxy(T,H), and quantum oscillations of the magnetization as a function of temperature at various magnetic fields for MoO2 with a monoclinic crystal structure. The band structure calculations show that MoO2 is a nodal-line semimetal when the spin-orbit coupling is ignored. It was found that a large magnetoresistance reaching 5.03 × 10^4% at 2 K and 9 T, its nearly quadratic field dependence, and a field-induced up-turn behavior of ρxx(T), the characteristics common for many topologically nontrivial as well as trivial semimetals, emerge also in MoO2. The observed properties are attributed to a perfect charge-carrier compensation, evidenced by both calculations relying on the Fermi surface topology and the Hall resistivity measurements. Both the observation of negative magnetoresistance for the magnetic field along the current ...

Latest version: v1
Publication date: Jan 09, 2021

Linear and quadratic magnetoresistance in the semimetal SiP2


Yuxing Zhou, Zhefeng Lou, Shengnan Zhang, Huancheng Chen, Qin Chen, Binjie Xu, Jianhua Du, Jinhu Yang, Hangdong Wang, Chuanying Xi, Li Pi, QuanSheng Wu, Oleg V. Yazyev, Minghu Fang

  • Multiple mechanisms for extremely large magnetoresistance (XMR) found in many topologically nontrivial/trivial semimetals have been theoretically proposed, but experimentally it is unclear which mechanism is responsible in a particular sample. In this paper, by the combination of band structure calculations, numerical simulations of magnetoresistance (MR), Hall resistivity, and de Haas-van Alphen (dHvA) oscillation measurements, we studied the MR anisotropy of SiP2 which is verified to be a topologically trivial, incomplete compensation semimetal. It was found that as magnetic field H is applied along the a-axis, the MR exhibits an unsaturated nearly linear H dependence, which was argued to arise from incomplete carriers compensation. For the H // [101] orientation, an unsaturated nearly quadratic H dependence of MR up to 5.88 × 10^4%(at 1.8 K, 31.2 T) and field-induced up-turn behavior in resistivity were observed, which was suggested due to the existence of hole open orbits ...

Latest version: v1
Publication date: Jan 09, 2021

Correlated states in twisted double bilayer graphene


Cheng Shen, Yanbang Chu, QuanSheng Wu, Na Li, Shuopei Wang, Yanchong Zhao, Jian Tang, Jieying Liu, Jinpeng Tian, Kenji Watanabe, Takashi Taniguchi, Rong Yang, Zi Yang Meng, Dongxia Shi, Oleg V. Yazyev, Guangyu Zhang

  • Electron–electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle. For bilayer graphene where the two layers are twisted by the ‘magic angle’, flat band and strong many-body effects lead to correlated insulating states and superconductivity. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of displacement field-tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find ...

Latest version: v1
Publication date: Jan 09, 2021

Multi-scale approach for the prediction of atomic scale properties


Andrea Grisafi, Jigyasa Nigam, Michele Ceriotti

  • Electronic nearsightedness is one of the fundamental principles that governs the behavior of condensed matter and supports its description in terms of local entities such as chemical bonds. Locality also underlies the tremendous success of machine-learning schemes that predict quantum mechanical observables -- such as the cohesive energy, the electron density, or a variety of response properties -- as a sum of atom-centred contributions, based on a short-range representation of atomic environments. One of the main shortcomings of these approaches is their inability to capture physical effects, ranging from electrostatic interactions to quantum delocalization, which have a long-range nature. Here we show how to build a multi-scale scheme that combines in the same framework local and non-local information, overcoming such limitations. We show that the simplest version of such features can be put in formal correspondence with a multipole expansion of permanent electrostatics. The ...

Latest version: v1
Publication date: Jan 07, 2021

High-performance NiOOH/FeOOH electrode for OER catalysis


Patrick Gono, Alfredo Pasquarello

  • The outstanding performance of NiOOH/FeOOH-based oxygen evolution reaction (OER) catalysts is rationalized in terms of a bifunctional mechanism involving two distinct active sites. In this mechanism, the OOH_ads reaction intermediate, which unfavorably affects the overall OER activity due to the linear scaling relationship, is replaced by O2 adsorbed at the active site on FeOOH, and H_ads adsorbed at the NiOOH substrate. Here, we use the computational hydrogen electrode method to assess promising models of both the FeOOH catalyst and the NiOOH hydrogen acceptor. These two materials are interfaced in various ways to evaluate their performance as bifunctional OER catalysts. In some cases, overpotentials as low as 0.16 V are found, supporting the bifunctional mechanism as a means to overcome the limitations imposed by linear scaling relationships.

Latest version: v1
Publication date: Jan 04, 2021

Emergence of nontrivial low-energy Dirac fermions in antiferromagnetic EuCd2As2


Junzhang Ma, Han Wang, Simin Nie, Changjiang Yi, Yuanfeng Xu, Hang Li, Jasmin Jandke, Wulf Wulfhekel, Yaobo Huang, Damien West, Pierre Richard, Alla Chikina, Vladimir Strocov, Joël Mesot, Hongming Weng, Shengbai Zhang, Youguo Shi, Tian Qian, Hong Ding, Ming Shi

  • When magnetism meets topology, colorful novel states can be created in materials. The realization of magnetic topological Dirac materials remains a major issue in topological physics studies. In this work, it is ascertained that the topologically nontrivial ground state of EuCd2As2 is a good candidate for different types of magnetic topological state: magnetic topological Dirac semimetal, axion insulator, antiferromagnetic TCI, and higher order topological insulator. This documents include all the raw data in the reference Advanced Materials 32, 1907565 (2020). This files contains all the raw data that published in the related paper. Fig.1.zip includes dat format transport measurement results which can be open by Origin. Fig.2.zip contains ibw format ARPES spectra which can be opened by IGOR. Fig.3.zip contains figures of the calculated FS, band structure. The dat format files can be open by IGOR. Fig.4.zip contains both ARPES spectra and STS data. sxm format file can be opened by ...

Latest version: v1
Publication date: Jan 02, 2021

Interpretations of ground-state symmetry breaking and strong correlation in wavefunction and density functional theories


John Perdew, Adrienn Ruzsinszky, Jianwei Sun, Niraj Nepal, Aaron Kaplan

  • Strong correlations within a symmetry-unbroken ground-state wavefunction can show up in approximate density functional theory as symmetry-broken spin-densities or total densities, which are sometimes observable. They can arise from soft modes of fluctuations (sometimes collective excitations) such as spin-density or charge-density waves at non-zero wavevector. In this sense, an approximate density functional for exchange and correlation that breaks symmetry can be more revealing (albeit less accurate) than an exact functional that does not. The examples discussed here include the stretched H2 molecule, antiferromagnetic solids, and the static charge-density wave/Wigner crystal phase of a low-density jellium. Time-dependent density functional theory is used to show quantitatively that the static charge density wave is a soft plasmon. More precisely, the frequency of a related density fluctuation drops to zero, as found from the frequency moments of the spectral function, calculated ...

Latest version: v3
Publication date: Dec 30, 2020

Molecular dynamics based cohesive law for epoxy-graphene interfaces


Jiadi Fan, Alexandros Anastassiou, Christopher Macosko, Ellad Tadmor

  • Molecular dynamics (MD) simulations are performed to obtain mode I and II fracture energies and cohesive laws for bulk epoxy and interfaces formed between epoxy and single-layer graphene (SLG), multilayer graphene (MLG), and multilayer graphene oxide (MLGO). The elastic moduli and ultimate tensile and shear strengths of epoxy--graphene interfaces are calculated from uniaxial tension and simple shear loadings. The results show that Young's modulus and the ultimate tensile strength increase relative to bulk epoxy, whereas the shear modulus and ultimate shear strength are reduced. Failure of epoxy--graphene interfaces in tension occurs due to the formation of voids in the epoxy. Failure in shear is due to tangential slipping at the interface. Under mixed mode conditions, the shear modulus and shear strength decrease with increasing tensile load. The critical energy release rate G_c for the studied epoxy--SLG/MLG/MLGO systems are obtained using a continuum fracture mechanics approach ...

Latest version: v1
Publication date: Dec 23, 2020

High performance Wannier interpolation of Berry curvature and related quantities with WannierBerri code


Stepan S. Tsirkin

  • The article presents a series of methods that boost the speed of Wannier interpolation by several orders of magnitude, as well as their implementation in the WannierBerri code. The present dataset contains input files, scripts, and the resulting data, which allow to reproduce the examples and figures published in the article. The current version of the code is also included.

Latest version: v1
Publication date: Dec 23, 2020

Tailoring interfacial properties in CaVO3 thin films and heterostructures with SrTiO3 and LaAlO3: A DFT+DMFT study


Sophie Beck, Claude Ederer

  • In this paper we use density functional theory combined with dynamical mean-field theory (DFT+DMFT) to study interface effects between the correlated metal CaVO3 and the two typical substrate materials SrTiO3 and LaAlO3. We find that the CaVO3/SrTiO3 interface has only a marginal influence on the CaVO3 thin film, with the dominant effect being the (bulklike) epitaxial strain imposed by the large lattice mismatch, rendering the CaVO3 film insulating due to the enhanced orbital polarization related to the strong level splitting between the t2g orbitals. In contrast, at the polar CaVO3/LaAlO3 interface, the presence of the interface can have a huge effect on the physical properties, depending both on the specific interface termination and on the specific boundary conditions imposed by the multilayer geometry. We compare three approaches to modeling the CaVO3/LaAlO3 interface, all of which impose a different set of (electrostatic) boundary conditions. Our results demonstrate that ...

Latest version: v1
Publication date: Dec 22, 2020

Calculation and interpretation of classical turning surfaces in solids


Aaron Kaplan, Stewart Clark, Kieron Burke, John Perdew

  • Classical turning surfaces of Kohn-Sham potentials separate classically-allowed regions (CARs) from classically-forbidden regions (CFRs). They are useful for understanding many chemical properties of molecules, but need not exist in solids, where the density never decays to zero. At equilibrium geometries, we find that CFRs are absent in perfect metals, rare in covalent semiconductors at equilibrium, but common in ionic and molecular crystals. In all materials, CFRs appear or grow as the internuclear distances are uniformly expanded. They can also appear at a monovacancy in a metal. Calculations with several approximate density functionals and codes confirm these behaviors. A classical picture of conduction suggests that CARs should be connected in metals, and disconnected in wide-gap insulators, and is confirmed in the limits of extreme compression and expansion. Surprisingly, many semiconductors have no CFR at equilibrium, a key finding for density functional construction. ...

Latest version: v1
Publication date: Dec 22, 2020

Identifying the trade-off between intramolecular singlet fission requirements in donor-acceptor copolymers


J. Terence Blaskovits, Maria Fumanal, Sergi Vela, Raimon Fabregat, Clemence Corminboeuf

  • Intramolecular singlet fission (iSF) has shown potential to improve the power conversion efficiency in photovoltaic devices by promoting the splitting of a photon-absorbing singlet exciton into two triplet excitons within a single molecule. Among different possibilities, the donor-acceptor modular strategy of copolymers has shown great promise in its ability to undergo iSF under certain conditions. However, the number of iSF donor-acceptor copolymers reported in the literature remains remarkably narrow and clear trends for the molecular design of better candidates have not yet been established. In this work, we identify the trade-off between the main iSF requirements of the donor-acceptor strategy and formulate design rules that allow them to be tuned simultaneously in a fragment-based approach. Based on a library of 2944 donor-acceptor copolymers, we establish simple guidelines to build promising novel materials for iSF. These consist in (1st) selecting an acceptor core with high ...

Latest version: v1
Publication date: Dec 22, 2020

Four- and twelve-band low-energy symmetric Hamiltonians and Hubbard parameters for twisted bilayer graphene using ab-initio input


Arkadiy Davydov, Kenny Choo, Mark H. Fischer, Titus Neupert

  • A computationally efficient workflow for obtaining low-energy tight-binding Hamiltonians for twisted bilayer graphene, obeying both crystal and time-reversal symmetries is presented in this work. The Hamiltonians at the first magic angle are generated using the Slater-Koster approach with parameters obtained by a fit to ab-initio data at larger angles. Low-energy symmetric four-band and twelve-band Hamiltonians are constructed using the Wannier90 software. The advantage of our scheme is that the low-energy Hamiltonians are purely real and are obtained with the maximum-localization procedure to reduce the spread of the basis functions. Finally, we compute extended Hubbard parameters for both models within the constrained random phase approximation (cRPA) for screening, which again respect the symmetries. The workflow is straightforwardly transferable to other twisted multi-layer materials.

Latest version: v1
Publication date: Dec 22, 2020

Local polarization in oxygen-deficient LaMnO3 induced by charge localization in the Jahn-Teller distorted structure


Chiara Ricca, Nicolas Niederhauser, Ulrich Aschauer

  • The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO3 (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the eg orbitals of the high-spin Mn3+ ions and leads to alternating long, short, and intermediate Mn-O bonds. Our DFT+U calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, like SrMnO3, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn-O bond is broken, affecting the d-orbital energies and leading to asymmetric and hence polar excess electron localization with ...

Latest version: v2
Publication date: Dec 22, 2020

Thermomechanical properties of honeycomb lattices from internal-coordinates potentials: the case of graphene and hexagonal boron nitrides


Francesco Libbi, Nicola Bonini, Nicola Marzari

  • Lattice dynamics in low-dimensional materials and, in particular, the quadratic behaviour of the flexural acoustic modes play a fundamental role in their thermomechanical properties. A first-principles evaluation of these can be very demanding, and can be affected by numerical noise that breaks translational or rotational invariance. In order to overcome these challenges, we study the Gartstein internal-coordinate potential and tune its 13 parameters on the first-principles interatomic force constants for graphene. We show that the resulting potential not only reproduces very well the phonon dispersions of graphene, but also those of carbon nanotubes of any diameter and chirality. The addition of a cubic term allows also to reproduce the dominant anharmonic terms, leading to a very good estimate of the lattice thermal conductivity. Finally, this potential form works very well also for boron nitride, provided it is fitted on the short-range (analytical) part of the interatomic ...

Latest version: v2
Publication date: Dec 22, 2020

A data-driven perspective on the colours of metal-organic frameworks


Kevin Maik Jablonka, Seyed Mohamad Moosavi, Mehrdad Asgari, Christopher Ireland, Luc Patiny, Berend Smit

  • Colour is at the core of chemistry and has been fascinating humans since ancient times. It is also a key descriptor of optoelectronic properties of materials and is used to assess the success of a synthesis. However, predicting the colour of a material based on its structure is challenging. In this work, we leverage subjective and categorical human assignments of colours to build a model that can predict the colour of compounds on a continuous scale, using chemically meaningful reasoning. In the process of developing the model, we also uncover inadequacies in current reporting mechanisms. For example, we show that the majority of colour assignments are subject to perceptive spread that would not comply with common printing standards. To remedy this, we suggest and implement an alternative way of reporting colour that is more suitable for a data-driven approach to materials science.

Latest version: v1
Publication date: Dec 22, 2020

Molecular mechanism of gas solubility in liquid: constant chemical potential molecular dynamics simulations


Narjes Ansari, Tarak Karmakar, Michele Parrinello

  • Accurate prediction of gas solubility in a liquid is crucial in many areas of chemistry, and a detailed understanding of the molecular mechanism of the gas solvation continues to be an active area of research. Here, we extend the idea of the constant chemical potential molecular dynamics (CμMD) approach to the calculation of the gas solubility in the liquid under constant gas chemical potential conditions. As a representative example, we utilize this method to calculate the isothermal solubility of carbon dioxide in water. Additionally, we provide microscopic insight into the mechanism of solvation that preferentially occurs in areas of the surface where the hydrogen network is broken.

Latest version: v1
Publication date: Dec 07, 2020

Controlling the quantum spin Hall edge states in two-dimensional transition metal dichalcogenides


Artem Pulkin, Oleg V. Yazyev

  • Two-dimensional transition metal dichalcogenides (TMDs) of Mo and W in their 1T′ crystalline phase host the quantum spin Hall (QSH) insulator phase. We address the electronic properties of the QSH edge states by means of first-principles calculations performed on realistic models of edge terminations of different stoichiometries. The QSH edge states show a tendency to have complex band dispersions and coexist with topologically trivial edge states. We nevertheless identify two stable edge terminations that allow isolation of a pair of helical edge states within the band gap of TMDs, with monolayer 1T′-WSe2 being the most promising material. We also characterize the finite-size effects in the electronic structure of 1T′-WSe2 nanoribbons. Our results provide guidance to the experimental studies and possible practical applications of QSH edge states in monolayer 1T′-TMDs.

Latest version: v1
Publication date: Dec 06, 2020

In situ high-energy X-ray diffraction of a CuZr-based metallic glass


Jiri Orava, Shanoob Balachandran, Xiaoliang Han, Olga Shuleshova, Ebrahim Nurouzi, Ivan Soldatov, Steffen Oswald, Olof Gutowski, Oleh Ivashko, Ann-Christin Dippel, Martin v. Zimmermann, Yurii P. Ivanov, A. Lindsay Greer, Dierk Raabe, Michael Herbig, Ivan Kaban

  • There is much current work on metallic glasses (MGs). The field is making rapid advances and has opened up questions of fundamental scientific interest. Metallic glasses are known to suffer from poor formability. Among other methods of improving the mechanical properties of MGs, introducing deformable crystalline phases into MGs is beneficial for enhancing the plastic compliance of MGs. The definition of the principal phase transformations (on heating and on cooling) underlying the feasibility of such a method is the focus of the deposited in situ high-energy XRD data carried out at Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany. This archive entry contains the temporal evolution of the equilibrium and metastable phases on flash-annealing (heating and cooling) and during containerless solidification via electromagnetic levitation with an unprecedented timescale of ~4 ms.

Latest version: v1
Publication date: Dec 05, 2020

Electronic transport across quantum dots in graphene nanoribbons: Toward built-in gap-tunable metal-semiconductor-metal heterojunctions


Kristiāns Čerņevičs, Oleg V. Yazyev, Michele Pizzochero

  • The success of all-graphene electronics is severely hindered by the challenging realization and subsequent integration of semiconducting channels and metallic contacts. Here, we comprehensively investigate the electronic transport across width-modulated heterojunctions consisting of a graphene quantum dot of varying lengths and widths embedded in a pair of armchair-edged metallic nanoribbons, of the kind recently fabricated via on-surface synthesis. We show that the presence of the quantum dot enables the opening of a width-dependent transport gap, thereby yielding built-in one-dimensional metal-semiconductor-metal junctions. Furthermore, we find that, in the vicinity of the band edges, the conductance is subject to a smooth transition from an antiresonant to a resonant transport regime upon increasing the channel length. These results are rationalized in terms of a competition between quantum-confinement effects and quantum dot-to-lead coupling. Overall, our work establishes ...

Latest version: v1
Publication date: Dec 04, 2020

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds


Nicolas Mounet, Marco Gibertini, Philippe Schwaller, Davide Campi, Andrius Merkys, Antimo Marrazzo, Thibault Sohier, Ivano E. Castelli, Andrea Cepellotti, Giovanni Pizzi, Nicola Marzari

  • Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozens of 2D materials have been successfully synthesized or exfoliated. Here, we search for novel 2D materials that can be easily exfoliated from their parent compounds. Starting from 108423 unique, experimentally known three-dimensional compounds we identify a subset of 5619 that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van-der-Waals density-functional theory, validated against experimental structural data and calculated random-phase-approximation binding energies, allow to identify 1825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 ...

Latest version: v4
Publication date: Dec 02, 2020

Pyrene-based metal organic frameworks


F. Pelin Kinik, Andres Ortega-Guerrero, Daniele Ongari, Christopher P. Ireland, Berend Smit

  • Pyrene is one of the most widely investigated aromatic hydrocarbons due to its unique optical and electronic properties. Hence, pyrene-based ligands have been investigated for the synthesis of metal-organic frameworks (MOFs) in the last few years. This dataset collects the atomic structures of the pyrene-based MOFs discussed in Table 1 of Ref. 1. The crystal structures have been manually curated to resolve partial occupancies and remove solvent molecules. Charge-neutral structures were optimized using DFT following the CURATED protocol described in 10.1021/acscentsci.9b00619.

Latest version: v2
Publication date: Dec 01, 2020

Fast Bayesian force fields from active learning: study of inter-dimensional transformation of stanene


Yu Xie, Jonathan Vandermause, Lixin Sun, Andrea Cepellotti, Boris Kozinsky

  • Gaussian process (GP) regression is one promising technique of constructing machine learning force fields with built-in uncertainty quantification, which can be used to monitor the quality of model predictions. A current limitation of existing GP force fields is that the prediction cost grows linearly with the size of the training data set, making accurate GP predictions slow. In this work, we exploit the special structure of the kernel function to construct a mapping of the trained Gaussian process model, including both forces and their uncertainty predictions, onto spline functions of low-dimensional structural features. This method is incorporated in the Bayesian active learning workflow for training of Bayesian force fields. To demonstrate the capabilities of this method, we construct a force field for stanene and perform large scale dynamics simulation of its structural evolution. We provide a fully open-source implementation of our method, as well as the training and testing examples with the stanene dataset.

Latest version: v3
Publication date: Dec 01, 2020

Small electron polarons in CsPbBr3


Nicklas Österbacka, Paul Erhart, Stefano Falletta, Alfredo Pasquarello, Julia Wiktor

  • We study the nature of excess electrons in CsPbBr3 and identify several single and double polaronic states. We emphasize the importance of proper inclusion of the self-interaction corrections for the stability of small electron polarons in this material. We demonstrate that spin–orbit coupling (SOC) has a significant impact on the energetics of the polaronic states. In particular, we find that SOC disfavors electron localization and leads to different polaronic geometries. Additionally, by carrying out thermodynamic integration, we show that small electron polarons are thermally stabilized in CsPbBr3. The small energy differences between the localized and delocalized electronic states could possibly reconcile the apparently conflicting properties of high charge-carrier mobilities and low recombinations rates.

Latest version: v1
Publication date: Dec 01, 2020

A Standard Solid State Pseudopotentials (SSSP) library optimized for precision and efficiency


Gianluca Prandini, Antimo Marrazzo, Ivano E. Castelli, Nicolas Mounet, Elsa Passaro, Nicola Marzari

  • Despite the enormous success and popularity of density functional theory, systematic verification and validation studies are still very limited both in number and scope. Here, we propose a universal standard protocol to verify publicly available pseudopotential libraries, based on several independent criteria including verification against all-electron equations of state and plane-wave convergence tests for phonon frequencies, band structure, cohesive energy and pressure. Adopting these criteria we obtain two optimal pseudopotential sets, namely the Standard Solid State Pseudopotential (SSSP) efficiency and precision libraries, tailored for high-throughput materials screening and high-precision materials modelling. As of today, the SSSP precision library is the most accurate open-source pseudopotential library available. This archive entry contains the database of calculations (phonons, cohesive energy, equation of state, band structure, pressure, etc.) together with the ...

Latest version: v5
Publication date: Nov 26, 2020

Reverse dark current in organic photodetectors and the major role of traps as source of noise


Jonas Kublitski, Andreas Hofacker, Bahman K. Boroujeni, Johannes Benduhn, Vasileios C. Nikolis, Christina Kaiser, Donato Spoltore, Hans Kleemann, Axel Fischer, Frank Ellinger, Koen Vandewal, Karl Leo

  • Organic photodetectors have promising applications in low-cost imaging, health monitoring and near infrared sensing. Recent research on organic photodetectors based on donor-acceptor systems has resulted in narrow-band, flexible and biocompatible devices, of which the best reach external photovoltaic quantum efficiencies approaching 100%. However, the high noise spectral density of these devices limits their specific detectivity to around 10^13 Jones in the visible and several orders of magnitude lower in the near-infrared, severely reducing performance. Here, we show that the shot noise, proportional to the dark current, dominates the noise spectral density, demanding a comprehensive understanding of the dark current. We demonstrate that, in addition to the intrinsic saturation current generated via charge-transfer states, dark current contains a major contribution from trap-assisted generated charges and decreases systematically with decreasing concentration of traps. By ...

Latest version: v1
Publication date: Nov 26, 2020

Oxynitride thin films versus particle-based photoanodes: a comparative study for photoelectrochemical solar water splitting


Fatima Haydous, Max Doebeli, Wenpig Si, Friedrich Waag, Fei Li, Ekaterina Pomjakushina, Alexander Wokaun, Bibal Gökce, Daniele Pergolesi, Thomas Lippert

  • The solar water splitting process assisted by semiconductor photocatalysts attracts growing research interests worldwide for the production of hydrogen as a clean and sustainable energy carrier. Due to their optical and electrical properties several oxynitride materials show great promise for the fabrication of efficient photocatalysts for solar water splitting. This study reports a comparative investigation of particle- and thin films-based photocatalysts using three different oxynitride materials. The absolute comparison of the photoelectrochemical activities favors the particle-based electrodes due to the better absorption properties and larger electrochemical surface area. However, thin films surpass the particle-based photoelectrodes due to their more suitable morphological features that improve the separation and mobility of the photo-generated charge carriers. Our analysis identifies what specific insights into the properties of materials can be achieved with the two complementary approaches.

Latest version: v1
Publication date: Nov 25, 2020

Improved photoelectrochemical water splitting of CaNbO2N photoanodes by Co-Pi photodeposition and surface passivation


Fatima Haydous, Wenping Si, Vitaly Guzenko, Friedrich Waag, Ekaterina Pomjakushina, Mario El Kazzi, Laurent Sévery, Alexander Wokaun, Daniele Pergolesi, Thomas Lippert

  • Photoelectrochemical solar water splitting is a promising approach to convert solar energy into sustainable hydrogen fuel using semiconductor electrodes. Due to their visible light absorption properties, oxynitrides have shown to be attractive photocatalysts for this application. In this study, the influence of the preparation method of CaNbO2N particles on their morphological and optical properties, and thereby their photoelectrochemical performance, is investigated. The best performing CaNbO2N photoanode is produced by ammonolysis of Nb enriched calcium niobium oxide. The enhanced photoactivity arises from an enlarged surface area and superior visible light absorption properties. The photoactivity of this photoanode was further enhanced by photodeposition of Co-Pi co-catalyst and by atomic layer deposition of an Al2O3 overlayer. A photocurrent density of 70 mA at 1.23 V vs RHE was achieved. The observed enhancement of the photoelectrochemical performance after Co-Pi/Al2O3 ...

Latest version: v1
Publication date: Nov 25, 2020

Yttrium tantalum oxynitride multiphases as photoanodes for water oxidation


Wenping Si, Zahra Pourmand-Tehrania, Fatima Haydous, Nicola Marzari, Ivano E. Castelli, Daniele Pergolesi, Thomas Lippert

  • Perovskite yttrium tantalum oxynitride is theoretically proposed as a promising semiconductor for solar water splitting because of the predicted bandgap and energy positions of band edges. In experiment, however, we show here that depending on processing parameters, yttrium tantalum oxynitrides exist in multiphases, including the desired perovskite YTaON2, defect fluorite YTa(O,N,o)4, and N-doped YTaO4. These multiphases have bandgaps ranging between 2.13 and 2.31 eV, all responsive to visible light. The N-doped YTaO4, perovskite main phase, and fluorite main phase derived from crystalline fergusonite oxide precursors exhibit interesting photoelectrochemical performances for water oxidation, while the defect fluorite derived from low crystallized scheelite-type oxide precursors show negligible activity. Preliminarily measurements show that loading IrOx cocatalyst on N-doped YTaO4 significantly improves its photoelectrochemical performance encouraging further studies to optimize this new material for solar fuel production.

Latest version: v1
Publication date: Nov 25, 2020

Suppressed charge recombination in hematite photoanode via protonation and annealing


Wenping Si, Fatima Haydous, Daniele Pergolesi, Thomas Lippert

  • Hematite as promising photoanode for solar water splitting suffers from severe bulk and surface charge recombination. This work describes that a protonation−annealing treatment can effectively suppress both bulk and surface charge recombination in hematite. Protons/electrons are electrochemically incorporated into hematite under 0.2 VRHE followed by annealing at 120 °C. The photocurrent density increases from ∼0.9 to 1.8 mA cm−2 at 1.23 VRHE under 1 sun, and further to 2.7 mA cm−2 after loading cobalt phosphate, stabilizing at round 2.4 mA cm−2. A cathodic shift of the onset potential of photocurrent is also observed. H2O2 oxidation, impedance spectroscopy, and Mott−Schottky measurements show that the protonation suppresses bulk recombination and enhances donor density, but introducing more surface recombination. The annealing reduces surface recombination, while preserving relatively high bulk charge separation efficiency. Different from previous reports on the electrochemically ...

Latest version: v1
Publication date: Nov 25, 2020

Large mobility modulation in ultrathin amorphous titanium oxide transistors


Nikhil Tiwale, Ashwanth Subramanian, Zhongwei Dai, Sayantani Sikder, Jerzy T. Sadowski, Chang-Yong Nam

  • Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick titanium oxide based ultrathin TFTs exhibiting 6-decade on-off ratio, predominantly driven by gate induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent ...

Latest version: v1
Publication date: Nov 13, 2020

Evaluation of photocatalysts for water splitting through combined analysis of surface coverage and energy-level alignment


Zhendong Guo, Francesco Ambrosio, Alfredo Pasquarello

  • To examine whether suitable conditions occur for the water splitting reaction at their interfaces with liquid water, we determine the pH-dependent surface coverage for a series of semiconductors, including GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For this we calculate acidity constants at surface sites through ab initio molecular dynamics simulations and a grand-canonical formulation of adsorbates. The resulting pH values at the point of zero charge show excellent agreement with experiment and thereby support the validity of our approach. By combining information concerning the surface coverage with the alignment of the band edges with respect to the relevant redox levels, we scrutinize the potential of the considered semiconductors as photocatalysts and identify the corresponding optimal pH ranges for hydrogen and oxygen evolution. More specifically, our results indicate that GaN stands out among these semiconductors as the most promising candidate for the overall ...

Latest version: v1
Publication date: Nov 13, 2020

CA-9, a dataset of carbon allotropes for training and testing of neural network potentials


Daniel Hedman, Tom Rothe, Gustav Johansson, Fredrik Sandin, J. Andreas Larsson, Yoshiyuki Miyamoto

  • The use of machine learning to accelerate computer simulations is on the rise. In atomistic simulations, the use of machine learning interatomic potentials (ML-IAPs) can significantly reduce computational costs while maintaining accuracy close to that of ab initio methods. To achieve this, ML-IAPs are trained on large datasets of images, meaning atomistic configurations labeled with data from ab initio calculations. Focusing on carbon, we have created a dataset, CA-9, consisting of 48000 images labeled with energies, forces and stress tensors obtained via ab initio molecular dynamics (AIMD). We use deep learning to train state-of-the-art neural network potentials (NNPs), a form of ML-IAP, on the CA-9 dataset and investigate how training and validation data can affect the performance of the NNPs. Our results show that image generation with AIMD causes a high degree of similarity between the generated images, which has a detrimental effect on the NNPs. However, by carefully choosing ...

Latest version: v1
Publication date: Nov 11, 2020

Self-consistent Hubbard parameters from density-functional perturbation theory in the ultrasoft and projector-augmented wave formulations


Iurii Timrov, Nicola Marzari, Matteo Cococcioni

  • The self-consistent evaluation of Hubbard parameters using linear-response theory is crucial for quantitatively predictive calculations based on Hubbard-corrected density-functional theory. Here, we extend a recently-introduced approach based on density-functional perturbation theory (DFPT) for the calculation of the on-site Hubbard U to also compute the inter-site Hubbard V. DFPT allows to reduce significantly computational costs, improve numerical accuracy, and fully automate the calculation of the Hubbard parameters by recasting the linear response of a localized perturbation into an array of monochromatic perturbations that can be calculated in the primitive cell. In addition, here we generalize the entire formalism from norm-conserving to ultrasoft and projector-augmented wave formulations, and to metallic ground states. After benchmarking DFPT against the conventional real-space Hubbard linear response in a supercell, we demonstrate the effectiveness of the present extended ...

Latest version: v1
Publication date: Nov 09, 2020

Li₄₋ₓGe₁₋ₓPₓO₄, a potential solid-state electrolyte for all-oxide microbatteries


Elisa Gilardi, Giuliana Materzanini, Leonid Kahle, Max Doebeli, Steven Lacey, Xi Cheng, Nicola Marzari, Daniele Pergolesi, Andreas Hintennach, Thomas Lippert

  • Solid-state electrolytes for Li-ion batteries are attracting growing interest as they allow building safer batteries, also using lithium-metal anodes. Here, we studied a compound in the lithium superionic conductor (LISICON) family, i.e. Li₄₋ₓGe₁₋ₓPₓO₄ (LGPO). Thin films were deposited via pulsed laser deposition, and their electrical properties were compared to those of ceramic pellets. A detailed characterization of their microstructures shows that thin films can be deposited fully crystalline at higher temperatures but also partially amorphous at room temperature. The conductivity is not strongly influenced by the presence of grain boundaries, exposure to air, or lithium deficiencies. First-principles molecular dynamics simulations were employed to calculate the lithium-ion diffusion profile and the conductivity at various temperatures of the ideal LGPO crystal. Simulations give the upper limit of conductivity for a defect-free crystal, which is in the range of 10–2 S cm–1 at ...

Latest version: v1
Publication date: Nov 06, 2020

3D ordering at the liquid–solid polar interface of nanowires


Mahdi Zamani, Giulio Imbalzano, Nicolas Tappy, Duncan T. L. Alexander, Sara Martí-Sánchez, Lea Ghisalberti, Quentin M. Ramasse, Martin Friedl, Gözde Tütüncüoglu, Luca Francaviglia, Sebastien Bienvenue, Cécile Hébert, Jordi Arbiol, Michele Ceriotti, Anna Fontcuberta i Morral

  • The nature of the liquid–solid interface determines the characteristics of a variety of physical phenomena, including catalysis, electrochemistry, lubrication, and crystal growth. Most of the established models for crystal growth are based on macroscopic thermodynamics, neglecting the atomistic nature of the liquid–solid interface. Here, experimental observations and molecular dynamics simulations are employed to identify the 3D nature of an atomic‐scale ordering of liquid Ga in contact with solid GaAs in a nanowire growth configuration. An interplay between the liquid ordering and the formation of a new bilayer is revealed, which, contrary to the established theories, suggests that the preference for a certain polarity and polytypism is influenced by the atomic structure of the interface. The conclusions of this work open new avenues for the understanding of crystal growth, as well as other processes and systems involving a liquid–solid interface.

Latest version: v1
Publication date: Nov 05, 2020

Direct, mediated and delayed intramolecular singlet fission mechanism in donor-acceptor copolymers


Maria Fumanal, Clémence Corminboeuf

  • Donor-acceptor (D-A) extended copolymers have shown great potential to be exploited for intramolecular Singlet Fission (iSF) because of their modular tunability and intrinsic ability to incorporate low-lying charge-transfer (CT) and a triplet-pair (TT) state. While the SF mechanism has been widely debated in homo- and hetero-dimers, little is known about the singlet splitting process in A-D-A copolymer trimers. Unlike traditional two-sites SF, the process of iSF in D-A copolymers involves three molecular units consisting of two A and one D following a A-D-A polymeric chain. This scenario is therefore, different from the homo-dimer analogous in terms of which states (if any) may drive the SF process. In this work, we identify how singlet splitting occurs in prototypical iSF D-A copolymer poly(benzodithiophene-alt-thiophene-1,1-dioxide) (BDT−TDO) by means of wave-packet propagations on the basis of the Linear Vibronic Coupling (LVC) model Hamiltonian. Our results reveal that three ...

Latest version: v1
Publication date: Nov 05, 2020

JuCLS database of core-level shifts from all-electron density functional theory simulations for chemical analysis of X-ray photoelectron spectra


Jens Bröder, Daniel Wortmann, Stefan Blügel

  • We present the JuCLS (Jülich core-level shifts) database which collects first principles calculations of core-level binding energies and core-level shifts (also known as chemical shifts). The calculations for this database were performed with the FLEUR program [1], a feature-full, freely available, open source FLAPW (full-potential linearized augmented planewave) code, based on density-functional theory. The FLAPW-method is a very accurate all-electron method which within density functional theory is universally applicable to all atoms of the periodic table. All calculations are run with AiiDA through workflows within the AiiDA-FLEUR package (version 0.12.3) [2]. Our database collects predicted core-level shifts, binding energies for X-ray photoelectron spectroscopy (XPS) and as a side product formation energies. Core-level shifts are calculated within the initial state approximation and binding energies are extracted from core-hole simulations. The JuCLS v1.0 contains initial ...

Latest version: v1
Publication date: Nov 05, 2020

RMapDB: chemical reaction route map data for quantum mechanical-based data chemistry


Hiroko Satoh, Tomohiro Oda, Kumiyo Nakakoji, Takeaki Uno, Satoru Iwata, Koichi Ohno

  • The record contains quantum mechanical (QM) global reaction route map (r-map) data. R-map is chemical reaction pathway networks, which compose equilibrium (EQ) and dissociation channel (DC) and transition state (TS) structures connected via intrinsic reaction coordinates (IRC) obtained by QM calculations. These are contents of RMapDB, which was published first in 2010 as a prototype and has been available online since 2014. The record contains global r-maps of three- to five-atom molecular systems. It contains also a conformational r-map of alpha-D-glucose and a global r-map of C2H4O2, which were discussed in potential energy surface-based conformational analysis and three-dimensional substructure search with the general root mean square deviation (G-RMSD) method. The RMapDB database has been developed in the RMap project for data chemistry based on QM chemical reaction route maps, together with an analytical tool RMapViewer and a server system RMapServer.

Latest version: v1
Publication date: Nov 04, 2020

A fourth-generation high-dimensional neural network potential with accurate electrostatics including non-local charge transfer


Tsz Wai Ko, Jonas A. Finkler, Stefan Goedecker, Jörg Behler

  • Machine learning potentials have become an important tool for atomistic simulations in many fields, from chemistry via molecular biology to materials science. Most of the established methods, however, rely on local properties and are thus unable to take global changes in the electronic structure into account, which result from long-range charge transfer or different charge states. In this work we overcome this limitation by introducing a fourth-generation high-dimensional neural network potential that combines a charge equilibration scheme employing environment-dependent atomic electronegativities with accurate atomic energies. The method, which is able to correctly describe global charge distributions in arbitrary systems, yields much improved energies and substantially extends the applicability of modern machine learning potentials. This is demonstrated for a series of systems representing typical scenarios in chemistry and materials science that are incorrectly described by ...

Latest version: v1
Publication date: Nov 04, 2020

Even–odd conductance effect in graphene nanoribbons induced by edge functionalization with aromatic molecules: basis for novel chemosensors


Kristiāns Čerņevičs, Michele Pizzochero, Oleg V. Yazyev

  • We theoretically investigate the electron transport in armchair and zigzag graphene nanoribbons (GNRs) chemically functionalized with p-polyphenyl and polyacene groups of increasing length. Our nearest-neighbor tight-binding calculations indicate that, depending on whether the number of aromatic rings in the functional group is even or odd, the resulting conductance at energies matching the energy levels of the corresponding isolated molecule is either unaffected or reduced by exactly one quantum as compared to the pristine GNR, respectively. Such an even–odd effect is shown to originate from a subtle interplay between the electronic states of the guest molecule that are spatially localized on the binding sites and those of the host nanoribbon. We next generalize our findings by employing more accurate tight-binding Hamiltonians along with density-functional theory calculations and critically discuss the robustness of the observed physical effects against the level of theory ...

Latest version: v1
Publication date: Nov 02, 2020

Learning on-top: regressing the on-top pair density for real-space visualization of electron correlation


Alberto Fabrizio, Ksenia R. Briling, David D. Girardier, Clemence Corminboeuf

  • The on-top pair density [Π(r)] is a local quantum chemical property, which reflects the probability of two electrons of any spin to occupy the same position in space. Simplest quantity related to the two-particles density matrix, the on-top pair density is a powerful indicator of electron correlation effects and, as such, it has been extensively used to combine density functional theory and multireference wavefunction theory. The widespread application of Π(r) is currently hindered by the need for post-Hartree-Fock or multireference computations for its accurate evaluation. In this work, we propose the construction of a machine learning model capable of predicting the CASSCF-quality on-top pair density of a molecule only from its structure and composition. Our model, trained on the GDB11-AD-3165 database, is able to predict with minimal error the on-top pair density of organic molecules bypassing completely the need for ab-initio computations. The accuracy of the regression is ...

Latest version: v1
Publication date: Oct 30, 2020

Optical imaging and spectroscopy of atomically precise armchair graphene nanoribbons


Sihan Zhao, Gabriela Borin Barin, Ting Cao, Jan Overbeck, Rimah Darawish, Tairu Lyu, Steve Drapcho, Sheng Wang, Tim Dumslaff, Akimitsu Narita, Michel Calame, Klaus Müllen, Steven G. Louie, Pascal Ruffieux, Roman Fasel

  • The record contains data that support the work where we report the optical imaging and absorption spectroscopy on atomically precise armchair graphene nanoribbons (GNRs) on insulating fused silica substrates. This is achieved by controlling light polarization on macroscopically aligned GNRs which greatly enhances the optical contrast of the submonolayer GNRs on the insulating substrates. We measure the linear absorption spectra of 7-armchair and 9-armchair GNRs in this study, and the experimental data agree qualitatively with ab inito calculation results. The polarization spectroscopy technique enables an unambiguous optical identification of GNRs and provides a rapid tool to characterize the transferred film over a large area.

Latest version: v1
Publication date: Oct 30, 2020

Pulay forces in density-functional theory with extended Hubbard functionals: from nonorthogonalized to orthogonalized manifolds


Iurii Timrov, Francesco Aquilante, Luca Binci, Matteo Cococcioni, Nicola Marzari

  • We present a derivation of the exact expression for Pulay forces in density-functional theory calculations augmented with extended Hubbard functionals, and arising from the use of orthogonalized atomic orbitals as projectors for the Hubbard manifold. The derivative of the inverse square root of the orbital overlap matrix is obtained as a closed-form solution of the associated Lyapunov (Sylvester) equation. The expression for the resulting contribution to the forces is presented in the framework of ultrasoft pseudopotentials and the projector-augmented-wave method, and using a plane wave basis set. We have benchmarked the present implementation with respect to finite differences of total energies for the case of NiO, finding excellent agreement. Owing to the accuracy of Hubbard-corrected density-functional theory calculations - provided the Hubbard parameters are computed for the manifold under consideration - the present work paves the way for systematic studies of solid-state and molecular transition-metal and rare-earth compounds.

Latest version: v1
Publication date: Oct 27, 2020

On‐surface synthesis of cumulene‐containing polymers via two‐step dehalogenative homocoupling of dibromomethylene-functionalized tribenzoazulene


José I. Urgel, Marco Di Giovannantonio, Kristjan Eimre, Thorsten G. Lohr, Junzhi Liu, Shantanu Mishra, Qiang Sun, Amogh Kinikar, Roland Widmer, Samuel Stolz, Max Bommert, Reinhard Berger, Pascal Ruffieux, Carlo A. Pignedoli, Klaus Müllen, Xinliang Feng, Roman Fasel

  • The record contains data that support our recent findings in the fabrication of cumulene containing polymers. Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on‐surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well‐controlled ultrahigh‐vacuum conditions. In the work we report the on‐surface synthesis of a polymer linked by cumulene‐like bonds on a Au(111) surface via sequential thermally activated dehalogenative C−C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene‐like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X‐ray photoelectron spectroscopy, complemented by density functional theory calculations.

Latest version: v1
Publication date: Oct 26, 2020

2‐D materials for ultrascaled field-effect transistors: one hundred candidates under the ab initio microscope


Cedric Klinkert, Aron Szabó, Christian Stieger, Davide Campi, Nicola Marzari, Mathieu Luisier

  • Due to their remarkable properties, single-layer 2-D materials appear as excellent candidates to extend Moore’s scaling law beyond the currently manufactured silicon FinFETs. However, the known 2-D semiconducting components, essentially transition metal dichalcogenides, are still far from delivering the expected performance. Based on a recent theoretical study that predicts the existence of more than 1800 exfoliable 2-D materials, we investigate here the 100 most promising contenders for logic applications.

Latest version: v1
Publication date: Oct 23, 2020

Pure Magnesium DFT calculations for interatomic potential fitting


Binglun Yin, Markus Stricker, W. A. Curtin

  • This dataset provides DFT (density functional theory as implemented in VASP, Vienna Ab Initio Simulation Package) calculations for pure Magnesium. It was designed by Binglun Yin, Markus Stricker and William A. Curtin for fitting a neural network potential with Behler-Parrinello symmetry functions. Binglun Yin carried out the calculation. It corresponds to a dataset that is commonly used to fit interatomic potentials for mechanics applications and includes structure-energy relationships for structures used to calculate: 1. Bulk properties 2. Generalized stacking fault energies 3. Decohesion and relaxed surfaces 4. Dimer 5. Corner and rod geometries 6. Vacancy formation energy

Latest version: v2
Publication date: Oct 22, 2020

Incipient antiferromagnetism in the Eu-doped topological insulator Bi2Te3


Philipp Rüßmann, Abdul Tcakaev, Volodymyr B. Zabolotnyy, Celso I. Fornari, Thiago R. F. Peixoto, Fabian Stier, Michael Dettbarn, Philipp Kagerer, Eugen Weschke, Enrico Schierle, Peter Bencok, Paulo H. O. Rappl, Eduardo Abramof, Hendrik Bentmann, Eberhard Goering, Friedrich Reinert, Vladimir Hinkov

  • Rare earth ions typically exhibit larger magnetic moments than transition metal ions and thus promise the opening of a wider exchange gap in the Dirac surface states of topological insulators. Yet, in a recent photoemission study of Eu-doped Bi2Te3 films, the spectra remained gapless down to T=20K. Here, we scrutinize whether the conditions for a substantial gap formation in this system are present by combining spectroscopic and bulk characterization methods with theoretical calculations. For all studied Eu doping concentrations, our atomic multiplet analysis of the M4,5 x-ray absorption and magnetic circular dichroism spectra reveals a Eu2+ valence and confirms a large magnetic moment, consistent with a 4f7 8S7/2 ground state. At temperatures below 10K, bulk magnetometry indicates the onset of antiferromagnetic (AFM) ordering. This is in good agreement with density functional theory, which predicts AFM interactions between the Eu impurities. Our results support the notion that ...

Latest version: v1
Publication date: Oct 22, 2020

Oxidation states, Thouless' pumps, and nontrivial ionic transport in nonstoichiometric electrolytes


Paolo Pegolo, Federico Grasselli, Stefano Baroni

  • Thouless’ quantization of adiabatic particle transport permits to associate an integer topological charge with each atom of an electronically gapped material. If these charges are additive and independent of atomic positions, they provide a rigorous definition of atomic oxidation states and atoms can be identified as integer-charge carriers in ionic conductors. Whenever these conditions are met, charge transport is necessarily convective, i.e. it cannot occur without substantial ionic flow, a transport regime that we dub trivial. We show that the topological requirements that allow these conditions to be broken are the same that would determine a Thouless’ pump mechanism if the system were subject to a suitably defined time-periodic Hamiltonian. The occurrence of these requirements determines a non-trivial transport regime whereby charge can flow without any ionic convection, even in electronic insulators. These results are first demonstrated with a couple of simple molecular ...

Latest version: v2
Publication date: Oct 21, 2020

Ab initio mobility of single-layer MoS2 and WS2: comparison to experiments and impact on the device characteristics


Youseung Lee, Sara Fiore, Mathieu Luisier

  • We combine the linearized Boltzmann Transport Equation (LBTE) and quantum transport by means of the Non-equilibrium Green's Functions (NEGF) to simulate monolayer MoS2 and WS2 ultra-scaled transistors with carrier mobilities extracted from experiments. Electron-phonon, charged impurity, and surface optical phonon scattering are taken into account with all necessary parameters derived from ab initio calculations or measurements, except for the impurity concentration. The LBTE method is used to scale the scattering self-energies of NEGF, which only include local interactions. This ensures an accurate reproduction of the measured mobilities by NEGF. We then perform device simulations and demonstrate that the considered transistors operate far from their performance limit (from 50% for MoS2 to 60% for WS2). Higher quality materials and substrate engineering will be needed to improve the situation.

Latest version: v1
Publication date: Oct 21, 2020

Unraveling the synergy between metal-organic frameworks and co-catalysts in photocatalytic water splitting


Stefano Falletta, Patrick Gono, Zhendong Guo, Stavroula Kampouri, Kyriakos C. Stylianou, Alfredo Pasquarello

  • We investigate the synergy occurring in photocatalytic water splitting between the metal-organic framework MIL-125-NH2 and two co-catalysts, namely NiO and Ni2P, by calculating their band edge alignment with respect to the redox levels of liquid water. For the NiO/H2O and Ni2P/H2O interfaces, we employ an explicit atomistic description of water and perform molecular dynamics simulations considering both molecular and dissociated water adsorbed at the co-catalyst surface. For the MIL-125-NH2/NiO and MIL-125-NH2/Ni2P interfaces, we rely on the concept of charge neutrality and use a scheme combining the electron affinities and the charge neutrality levels of the interface components. We provide a description of the underlying fundamental processes that is consistent with photoluminescence and intrinsic activity experiments and that supports NiO and Ni2P as suitable co-catalysts for MIL-125-NH2 as far as the hydrogen evolution reaction is concerned.

Latest version: v1
Publication date: Oct 14, 2020

Evidence of large polarons in photoemission band mapping of the perovskite semiconductor CsPbBr3


Michele Puppin, Serhii Polishchuk, Nicola Colonna, Alberto Crepaldi, Dmitry Dirin, Olga Nazarenko, Riccardo De Gennaro, Gianmarco Gatti, Silvan Roth, Thomas Barillot, Luca Poletto, Rui Patrick Xian, Laurenz Rettig, Martin Wolf, Ralph Ernstorfer, Maksym V. Kovalenko, Nicola Marzari, Marco Grioni, Majed Chergui

  • Lead-halide perovskite (LHP) semiconductors are emergent optoelectronic materials with outstanding transport properties which are not yet fully understood. We find signatures of large polaron formation in the electronic structure of the inorganic LHP CsPbBr3 by means of angle-resolved photoelectron spectroscopy. The experimental valence band dispersion shows a hole effective mass of 0.26±0.02 me, 50% heavier than the bare mass m0=0.17 me predicted by ab-initio calculations. Calculations of the electron-phonon coupling indicate that phonon dressing of the carriers mainly occurs via distortions of the Pb-Br bond with a Fröhlich coupling parameter α=1.82. A good agreement between theoretical and experimental data is obtained within the Feynman polaron model, validating a viable theoretical method to predict the carrier effective mass of LHPs ab initio.

Latest version: v1
Publication date: Oct 14, 2020

Bathochromic shift in the UV-visible absorption spectra of phenols at ice surfaces: insights from first-principles calculations


Fernanda C. Bononi, Zekun Chen, Ted Hullar, Dario Rocca, Oliviero Andreussi, Cort Anastasio, Davide Donadio

  • Some organic pollutants in snowpack undergo faster photodegradation than in solution. One possible explanation for such effect is that their UV-visible absorption spectra are shifted toward lower energy when the molecules are adsorbed at the air-ice interface. However, such bathochromic shift is difficult to measure experimentally. Here we employ a multiscale/multimodel approach that combines classical and first-principles molecular dynamics, quantum chemical methods and statistical learning to compute the light absorption spectra of two phenolic molecules in different solvation environments at the relevant thermodynamic conditions. Our calculations provide an accurate estimate of the bathochromic shift of the lowest-energy UV-visible absorption band when these molecules are adsorbed at the air-ice interface, and they shed light into its molecular origin.

Latest version: v1
Publication date: Oct 14, 2020

Collective all‐carbon magnetism in triangulene dimers


Shantanu Mishra, Doreen Beyer, Kristjan Eimre, Ricardo Ortiz, Joaquín Fernández‐Rossier, Reinhard Berger, Oliver Gröning, Carlo A. Pignedoli, Roman Fasel, Xinliang Feng, Pascal Ruffieux

  • This record contain data to support the result we published in the work "Collective All‐Carbon Magnetism in Triangulene Dimers". Triangular zigzag nanographenes, such as triangulene and its π‐extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building blocks for high‐spin networks with long‐range magnetic order, which are of immense fundamental and technological relevance. In the publication we present the on‐surface synthesis and a proof‐of‐principle experimental study of magnetism in covalently bonded triangulene dimers. On‐surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4‐phenylene spacer.

Latest version: v1
Publication date: Oct 14, 2020

On-surface synthesis of non-benzenoid nanographenes by oxidative ring-closure and ring-rearrangement reactions


Thorsten G. Lohr, José I. Urgel, Kristjan Eimre, Junzhi Liu, Marco Di Giovannantonio, Shantanu Mishra, Reinhard Berger, Pascal Ruffieux, Carlo A. Pignedoli, Roman Fasel, Xinliang Feng

  • In this record we provide data supporting our recent results discussed in the fabrication of non-benzenoid nanographenes. Nanographenes (NGs) have gained increasing attention due to their immense potential as tailor-made organic materials for nanoelectronics and spintronics. They exhibit a rich spectrum of physicochemical properties that can be tuned by controlling the size or the edge structure or by introducing structural defects in the honeycomb lattice. In the published manuscript we report the design and on-surface synthesis of NGs containing several odd-membered polycycles induced by a thermal procedure on Au(111). Our scanning tunneling microscopy, noncontact atomic force microscopy, and scanning tunneling spectroscopy measurements, complemented by computational investigations, describe the formation of two nonbenzenoid NGs (2A,B) containing four embedded azulene units in the polycyclic framework, via on-surface oxidative ring-closure reactions. Interestingly, we observe ...

Latest version: v1
Publication date: Oct 14, 2020

Reaction pathway towards 7-atom-wide armchair graphene nanoribbon formation and identification of intermediate species on Au(111)


Sebastian Thussing, Sebastian Flade, Kristjan Eimre, Carlo A. Pignedoli, Roman Fasel, Peter Jakob

  • In this record we provide data supporitng our recent results discussed in the characterization of the fabrication process of graphene nanoribbons. The prototypical surface reaction of 10,10′-dibromo-9,9′-bianthryl (DBBA) toward the seven-atom-wide armchair graphene nanoribbon (7-AGNR) on the Au(111) surface has been investigated by means of vibrational spectroscopy, thermal desorption spectroscopy, and density functional theory. Specifically, a direct correlation between annealing temperature and the formation of various intermediate species is derived. By comparison of IR spectra with results from DFT calculations, an identification of reaction intermediates has been achieved, allowing for a precise mapping of individual reaction steps. Thereby, we identify a prior unknown partially dehalogenated and strongly tilted DBBA* monoradical species (DBBA-1Br) after mild annealing (380–450 K). This inclined adsorption geometry stabilizes the second Br atom, preventing full ...

Latest version: v1
Publication date: Oct 14, 2020

Machine learning for metallurgy: a neural network potential for Al-Cu


Daniel Marchand, Abhinav Jain, Albert Glensk, W. A. Curtin

  • High-strength metal alloys achieve their performance via careful control of precipitates and solutes. The nucleation, growth, and kinetics of precipitation, and the resulting mechanical properties, are inherently atomic-scale phenomena, particularly during early-stage nucleation and growth. Atomistic modeling using interatomic potentials is a desirable tool for understanding the detailed phenomena involved in precipitation and strengthening, which requires length and time scales far larger than those accessible by first-principles methods. Current interatomic potentials for alloys are not, however, sufficiently accurate for such studies. Here, a family of neural-network potentials (NNPs) for the Al-Cu system is presented as a first example of a machine-learning potential that can achieve near-first-principles accuracy for many different metallurgically-important aspects of this alloy. High fidelity predictions of intermetallic compounds, elastic constants, dilute ...

Latest version: v3
Publication date: Oct 14, 2020

Effect of charge self-consistency in DFT+DMFT calculations for complex transition metal oxides


Alexander Hampel, Sophie Beck, Claude Ederer

  • We investigate the effect of charge self-consistency (CSC) in density-functional theory plus dynamical mean-field theory calculations compared to simpler “one-shot” calculations for materials where interaction effects lead to a strong redistribution of electronic charges between different orbitals or between different sites. We focus on two systems close to a metal-insulator transition (MIT), for which the importance of CSC is currently not well understood. Specifically, we analyze the strain-related orbital polarization in the correlated metal CaVO3 and the spontaneous electronic charge disproportionation in the rare-earth nickelate LuNiO3. In both cases, we find that the CSC treatment reduces the charge redistribution compared to cheaper one-shot calculations. However, while the MIT in CaVO3 is only slightly shifted due to the reduced orbital polarization, the effect of the site polarization on the MIT in LuNiO3 is more subtle. Furthermore, we highlight the role of the ...

Latest version: v1
Publication date: Oct 12, 2020

On-surface synthesis of unsaturated carbon nanostructures with regularly fused pentagon-heptagon pairs


Ian Cheng-Yi Hou, Qiang Sun, Kristjan Eimre, Marco Di Giovannantonio, José I. Urgel, Pascal Ruffieux, Akimitsu Narita, Roman Fasel, Klaus Müllen

  • In this record we provide data to support our recent findings for the fabrication of Unsaturated Carbon Nanostructures with Regularly Fused Pentagon–Heptagon Pairs. Multiple fused pentagon–heptagon pairs are frequently found as defects at the grain boundaries of the hexagonal graphene lattice and are suggested to have a fundamental influence on graphene-related materials. However, the construction of sp2-carbon skeletons with multiple regularly fused pentagon–heptagon pairs is challenging. In this work, we found that the pentagon–heptagon skeleton of azulene was rearranged during the thermal reaction of an azulene-incorporated organometallic polymer on Au(111). The resulting sp2-carbon frameworks were characterized by high-resolution scanning probe microscopy techniques and feature novel polycyclic architectures composed of multiple regularly fused pentagon–heptagon pairs. Moreover, the calculated analysis of its aromaticity revealed a peculiar polar electronic structure.

Latest version: v1
Publication date: Oct 12, 2020

Coupled spin states in armchair graphene nanoribbons with asymmetric zigzag edge extensions


Qiang Sun, Xuelin Yao, Oliver Gröning, Kristjan Eimre, Carlo A. Pignedoli, Klaus Müllen, Akimitsu Narita, Roman Fasel, Pascal Ruffieux

  • In this record we provide data supporting our recent work on coupled spin states in armchair nanoribbons. Exact positioning of sublattice imbalanced nanostructures in graphene nanomaterials offers a route to control interactions between induced local magnetic moments and to obtain graphene nanomaterials with magnetically nontrivial ground states. Our results reveal that such sublattice imbalanced nanostructures can be incorporated along a large band gap armchair graphene nanoribbon on the basis of asymmetric zigzag edge extensions, achieved by incorporating specifically designed precursor monomers. Scanning tunneling spectroscopy of an isolated and electronically decoupled zigzag edge extension reveals Hubbard-split states in accordance with theoretical predictions. Mean-field Hubbard-based modeling of pairs of such zigzag edge extensions reveals ferromagnetic, antiferromagnetic, or quenching of the magnetic interactions depending on the relative alignment of the asymmetric edge ...

Latest version: v1
Publication date: Oct 12, 2020

On-surface synthesis of oligo(indenoindene)


Marco Di Giovannantonio, Qiang Chen, José I. Urgel, Pascal Ruffieux, Carlo A. Pignedoli, Klaus Müllen, Akimitsu Narita, Roman Fasel

  • In this record we provide data to support our recent work on the synthesis of oligo(indenoindene). Fully conjugated ladder polymers (CLP) possess unique optical and electronic properties and are considered promising materials for applications in (opto)electronic devices. Poly(indenoindene) is a CLP consisting of an alternating array of five- and six-membered rings, which has remained elusive so far. Our results relate to on-surface synthesis of oligo(indenoindene) on Au(111). Its structure and a low electronic band gap have been elucidated by low-temperature scanning tunneling microscopy and spectroscopy and noncontact atomic force microscopy, complemented by density functional theory calculations. Achieving defect-free segments of oligo(indenoindene) offers exclusive insight into this CLP and provides the basis to further synthetic approaches.

Latest version: v1
Publication date: Oct 12, 2020

Ab initio modeling of thermal transport through van der Waals materials


Sara Fiore

  • An advanced modeling approach is presented to shed light on the thermal transport properties of van der Waals materials (vdWMs) composed of single-layer transition metal dichalcogenides (TMDs) stacked on top of each other with a total or partial overlap only in the middle region. It relies on the calculation of dynamical matrices from first principles and on their usage in a phonon quantum transport simulator. We observe that vibrations are transferred microscopically from one layer to the other along the overlap region which acts as a filter selecting out the states that can pass through it. Our work emphasizes the possibility of engineering heat flows at the nanoscale by carefully selecting the TMD monolayers that compose vdWMs.

Latest version: v1
Publication date: Oct 09, 2020

The role of water in host-guest interaction


Valerio Rizzi, Luigi Bonati, Narjes Ansari, Michele Parrinello

  • One of the main applications of atomistic computer simulations is the calculation of ligand binding free energies. The accuracy of these calculations depends on the force field quality and on the thoroughness of configuration sampling. Sampling is an obstacle in simulations due to the frequent appearance of kinetic bottlenecks in the free energy landscape. Very often this difficulty is circumvented by enhanced sampling techniques. Typically, these techniques depend on the introduction of appropriate collective variables that are meant to capture the system's degrees of freedom. In ligand binding, water has long been known to play a key role, but its complex behaviour has proven difficult to fully capture. In this paper we combine machine learning with physical intuition to build a non-local and highly efficient water-describing collective variable. We use it to study a set of of host-guest systems from the SAMPL5 challenge. We obtain highly accurate binding free energies and good ...

Latest version: v1
Publication date: Sep 28, 2020

Full daytime sub-ambient radiative cooling in commercial-like paints with high figure of merit


Xiangyu Li, Joseph Peoples, Zhifeng Huang, Zixuan Zhao, Jun Qiu, Xiulin Ruan

  • Radiative cooling is a passive cooling technology by reflecting sunlight and emitting radiation in the sky window. Although highly desired, full daytime sub-ambient radiative cooling in commercial-like single-layer particle-matrix paints is yet to be achieved. Here we demonstrate full daytime sub-ambient radiative cooling in CaCO3-acrylic paint by utilizing the large bandgap CaCO3 fillers, a high particle concentration of 60% and a broad size distribution. Our paint shows high solar reflectance of 95.5% and high normal emissivity of 0.94 in the sky window. Field tests show cooling power exceeding 37W/m2 and surface temperature more than 1.7˚C below ambient at noon. A figure of merit RC is proposed to compare the cooling performance independent of weather conditions. The standard RC of our paint is 0.49, among the best radiative cooling performance while offering unprecedented benefits of the convenient paint form, low cost, and the compatibility with commercial paint fabrication process.

Latest version: v1
Publication date: Sep 25, 2020

Randomly-displaced methane configurations


Sergey Pozdnyakov, Michael Willatt, Michele Ceriotti

  • Most of the datasets to benchmark machine-learning models contain minimum-energy structures, or small fluctuations around stable geometries, and focus on the diversity of chemical compositions, or the presence of different phases. This dataset provides a large number (7732488) configurations for a simple CH4 composition, that are generated in an almost completely unbiased fashion. Hydrogen atoms are randomly distributed in a 3A sphere centered around the carbon atom, and the only structures that are discarded are those with atoms that are closer than 0.5A, or such that the reference DFT calculation does not converge. This dataset is ideal to benchmark structural representations and regression algorithms, verifying whether they allow reaching arbitrary accuracy in the data rich regime.

Latest version: v2
Publication date: Sep 18, 2020

Charge separation and charge carrier mobility in photocatalytic metal-organic frameworks


Maria Fumanal, Andres Ortega-Guerrero, Kevin Maik Jablonka, Berend Smit, Ivano Tavernelli

  • Metal-Organic Frameworks (MOFs) are highly versatile materials owing to their vast structural and chemical tunability. These hybrid inorganic-organic crystalline materials offer an ideal platform to incorporate light-harvesting and catalytic centers and thus, exhibit a great potential to be exploited in solar-driven photocatalytic processes such as H2 production and CO2 reduction. To be photocatalytically active, UV-visible optical absorption and appropriate band alignment with respect to the target redox potential is required. Despite fulfilling these criteria, the photocatalytic performance of MOFs is still limited by their ability to produce long-lived electron-hole pairs and long-range charge transport. In this work, we present a computational strategy to address these two descriptors in MOF structures and translate them into charge transfer numbers and effective mass values. We apply our approach to 15 MOF structures from the literature that encompass the main strategies used ...

Latest version: v1
Publication date: Sep 17, 2020

Accurate optical spectra through time-dependent density functional theory based on screening-dependent hybrid functionals


Alexey Tal, Peitao Liu, Georg Kresse, Alfredo Pasquarello

  • We investigate optical absorption spectra obtained through time-dependent density functional theory (TD-DFT) based on nonempirical hybrid functionals that are designed to correctly reproduce the dielectric function. The comparison with state-of-the-art GW calculations followed by the solution of the Bethe-Salpeter equation (BSE-GW) shows close agreement for both the transition energies and the main features of the spectra. We confront TD-DFT with BSE-GW by focusing on the model dielectric function and the local exchange-correlation kernel. The present TD-DFT approach achieves the accuracy of BSE-GW at a fraction of the computational cost.

Latest version: v1
Publication date: Sep 11, 2020

First-principles simulation of electron transport and thermoelectric property of materials, including electron-phonon scattering, defect scattering, and phonon drag


Jiawei Zhou, Te-Huan Liu, Qichen Song, Qian Xu, Zhiwei Ding, Bolin Liao, Gang Chen

  • We publish our code for electron transport and thermoelectric property calculations in solid state materials. This code is modified based on the EPW v4 code, originally from the open-source Quantum ESPRESSO suite (version 5.4.0), and is released under GNU General Public License. The original EPW v4 is developed by S. Poncé, E.R. Margine, C. Verdi, and, F. Giustino, initially released inside Quantum ESPRESSO in 2016. This modified version is dedicated to the simulation of electron-phonon transport properties in quantum materials. Specifically, it calculates the electron-phonon and electron-defect scattering rates and uses them as inputs in Boltzmann transport equation to obtain transport properties (e.g. electrical conductivity, mobility, Seebeck coefficient, thermoelectric power factor, and electronic thermal conductivity).

Latest version: v1
Publication date: Sep 08, 2020

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