1) Usage of the "QMspin database Part 1":
  
   Details on the construction of the "QMspin data base Part 1" and its scientific
   discussion is published here: "Large yet bounded: Spin gap ranges in carbenes"
   Max Schwilk, Diana N. Tahchieva, O. Anatole von Lilienfeld
   https://arxiv.org/abs/2004.10600. We would greatly appreciate if this publication is
   acknowledged when making usage of the QMspin database. Once published in a scientific
   peer-reviewed journal, we would prefer if the journal article citation is used instead.


2) Nomenclature of the QMspin structures:
   
   The structures have the same primary index as their QM9 database parent molecule. See
   "Quantum chemistry structures and properties of 134 kilo molecules" Raghunathan
   Ramakrishnan, Pavlo O. Dral, Matthias Rupp, O. Anatole von Lilienfeld, Sci. Data 1,
   140022 (2014) for details on the QM9 data base. A secondary index enumerates the
   carbenes derived from a certain parent QM9 molecule. Note that there might be "gaps"
   in this index as not all possible carbenes for a given QM9 molecule have been retained
   in the data base. A third index indicates the optimized spin state ("t" or "s"). There
   are 7958 triplet state structures and 5022 singlet state structures. Among all these,
   for 3775 molecules both spin state structures are available, for which in consequence
   adiabatic spin gaps are available. For 269 singlet state structures and 6255 triplet
   state structures harmonic frequencies have been computed, all of which are real-valued,
   i. e. describe a local minimum on the PES.
   The triplet state structures are optimized with open shell restricted B3LYP/def2-TZVP
   using the QM9 molecule as initial geometries. The singlet state structures are optimized
   with CASSCF(2e,2o)/vdz-f12 using the triplet state structures as initial geometries. 
   The single point spin gap energies are computed with MRCISD+Q-F12/vdz-f12 using state-
   averaged CASSCF(2e,2o) as a reference wave function. For all structures it has been
   verified that the two non-bonding active space orbitals are well localized on the carbene
   center. For details see "Large yet bounded: Spin gap ranges in carbenes" Max Schwilk,
   Diana N. Tahchieva, O. Anatole von Lilienfeld https://arxiv.org/abs/2004.10600. 
  
 
3) Structure of the QMspin database:
   
   The database is provided as gzipped tarball. It contains the following folders and files:
   
   - "qmspin_data_overview.csv":
     The file contains a list of all the structure names along with computed vertical and
     adiabatic spin gaps computed at CASSCF and MRCI level of theory in CSV format with 
     "," as delimiter. "Spin gap" means singlet state absolute energy minus triplet
     state absolute energy. A "None" entry indicates that a certain spin gap has not been
     computed for that molecule. Furthermore, it is indicated if the harmonic frequencies
     of the spin state structures of a molecule have been calculated. The first line of the
     file indicates the data type of each column as an abbreviation. By order, the columns
     contain:
         i)    "MOL": Molecule index
         ii)   "CVS": CASSCF vertical spin gap of the singlet state structure in kcal/mol
         iii)  "MVS": MRCI vertical spin gap of the singlet state structure in kcal/mol
         iv)   "CVT": CASSCF vertical spin gap of the triplet state structure in kcal/mol
         v)    "MVT": MRCI vertical spin gap of the triplet state structure in kcal/mol
         vi)   "CA" : CASSCF adiabatic spin gap in kcal/mol
         vii)  "MA" : MRCI adiabatic spin gap in kcal/mol
         viii) "FS" : "yes"/"no" if singlet state structure harmonic frequencies have been
                      computed, the wave number in cm^{-1} of the lowest frequency is given
                      if it is below 50 (the latter implies "yes")
         ix)   "FT" : "yes"/"no" if triplet state structure harmonic frequencies have been
                      computed, the wave number in cm^{-1} of the lowest frequency is given
                      if it is below 50 (the latter implies "yes")
     
   - "outputs_singlet_sp":
     Raw outputs of the singlet state structure single point spin gap calculations. A few
     of the outputs contain also the previous CASSCF geometry optimization.
     
   - "outputs_triplet_sp":
     Raw outputs of the triplet state structure single point spin gap calculations. 
     
   - "outputs_singlet_freq":
     Raw outputs of the singlet state frequency calculations.

   - "outputs_triplet_freq":
     Raw outputs of the triplet state frequency calculations.

   - "geometries_singlet":
     xyz files of the singlet state optimized structures with "," as delimiter. The comment
     line contains the computed absolute energies in Hartree of the structure at CASSCF and
     MRCI level of theory along with indications on the used basis sets.

   - "geometries_triplet":
     Analog to description "geometries_singlet".

   - "frequencies_singlet":
     xyz-style files of the singlet state optimized structures that contain the cartesian
     coordinates in the first three columns as well as additional columns with the vibrational
     modes in [Angstrom] (i. e. not weighted by the square root of the atomic mass) given in
     the same cartesian coordinate basis. Hence there are three columns per normal mode.
     Column delimiter is ",". Frequencies and intensities (as computed from the nuclear
     transition dipole moment of the fundamental vibrational excition v=0 to v=1) of the normal
     modes are given with units specified in the file ordered by increasing wave number.
     Modes with wave numbers below 50 cm^{-1} are not given. Further details on the
     frequency calculations, including dipole moments and first order derivatives of the dipole
     moments are given in the corresponding output files in the folder "outputs_singlet_freq".

   - "frequencies_triplet":
     Analog to description "frequencies_singlet".