Summary of the CCCBDB
The NIST Computational Chemistry Comparison and Benchmark Database is a
collection of experimental and ab initio thermochemical properties for a
selected set of molecules.
- The goals are:
- 1) Provide a benchmark set of molecules for the evaluation of ab initio
- 2) Allow the comparison between different ab initio computational methods
for the prediction of thermochemical properties.
- The thermochemical values with error bars included in the CCCBDB are:
- 1. Enthalpies of formation.
- 2. Entropies, heat corrections (integrated heat capacity).
- 3. Data needed to compute thermochemical properties, such as geometries, rotational constants,
vibrational frequencies, barriers to internal rotation, and electronic energy levels.
- 4. Additional computed properties such as atomic charges, electric dipole moments, quadrupole moments, polarizabilities, and
HOMO-LUMO gaps. See the index (Section XIX. Index of Properties)
for more properties.
The calculations are ongoing. Over 250000 have been completed.
Model chemistries included so far:
|Hartree Fock methods
|Moller Plesset perturbation methods
||MP2, MP2=FULL, MP3, MP4
|partial configuration interaction
|quadratic configuration interaction
|coupled cluster methods
||CCD, CCSD, CCSD(T), CCSD(T)=FULL
|Density functional methods
||BLYP, B1B95, B3LYP, B3PW91, MPW1PW91, PBE
||G1, G2, G2MP2, G3, G3B3, CBS-Q
- The molecules to be included have been selected from previous collections,
such as the G2 data set [Larry A. Curtiss, Krishnan Raghavachari, Gary
W. Trucks et al., JCP 94 (11), 7221 (1991)] and the G2 extended data
set [Larry A. Curtiss, Krishnan Raghavachari, Paul C. Redfern et al.,
J. Chem. Phys. 106 (3), 1063 - 1079 (1997)]. This set has been
augmented from the NIST
Chemistry Webbook using the following criteria:
- The error bars on the Enthalpy of formation are less than or equal to 8
- The species contains no atoms with atomic number greater than 17
- The species contains six or fewer heavy atoms and twenty or fewer total
atoms. This allows species as large as hexane.
- The above constraints have been loosened to include some larger species
(substituted benzenes, cyclooctatetraene, S8, naphthalene, adamantane)
and some Br-containing and Se-containing molecules,
as well as a few Cu-containing molecules.
See sections I.B.1.b or I.B.1.c for lists of the
molecules sorted by number of atoms or by number of heavy atoms.
The experimental thermochemical data have been obtained primarily from the following
- CODATA [J. D. Cox, D. D. Wagman, and V. A. Medvedev, CODATA Key values for
Thermodynamics (Hemisphere, New York, 1989)] This is a short list of species
with internationally accepted values for enthalpies of formation, entropies and
heat corrections. Provides error bars for all three properties.
- JANAF [M. W. Chase, Jr., C. A. Davies, J. R. Downey, Jr. et al., Journal
of Physical and Chemical Reference Data 14 Supplement No. 1 (1985)] This is
an evaluated list of mostly inorganic species. Provides error bars for
enthalpies and entropies.
- Gurvich [L. V. Gurvich, I. V. Veyts, and C. B. Alcock, Thermodynamic
Properties of Individual Substances (Hemisphere Publishing Corporation, New
York, New York, 1989)] Evaluated. Small organics. Error bars for enthalpy.
- TRC [Michael Frenkel, G. J. Kabo, K. N. Marsh et al., Thermodynamics of
Organic Compounds in the Gas State (Thermodynamics Research Center, College
Station, Texas, 1994)] Evaluated. No error bars.
- Other sources on individual species where noted.
- General discrepancy between measured and calculated data.
- 1. Enthalpies of formation are not measured. The properties determined
experimentally are usually Enthalpies of reaction at some temperature above 0 K.
The ab initio methods provide an absolute energy (relative to compete
ionization of all atoms) at 0 K. To convert from one temperature to another,
one uses the integrated heat capacity (heat correction). Ideally the
procedure and supporting data used to convert from the experimental
measurement to a Enthalpy of formation would be available in the database. This
is true for the ab initio determinations as well, where the simplest
property to calculate is the atomization energy of a molecule at 0 K.
- Interconnectedness of data.
- 1. Geometries are needed to determine rotational constants, which are
needed for heat corrections and entropies. Geometries are also useful as
starting input for ab initio calculations. Experimental geometries can be
compared with ab initio geometries to evaluate ab initio methods.
- 2. Vibrational frequencies are needed for heat corrections and entropies.
- 3. Some Enthalpies of formation are derived from measured equilibrium, this
derivation requires an entropy. If the vibrational frequencies come from ab
initio methods, the Enthalpy of formation is not purely an experimental
- Lack of experimental data
- 1. The integrated heat capacity, or heat correction, is used to convert
the enthalpy of formation from one temperature to another. It is calculated
using standard statistical mechanics, using information such as mass,
rotational constants, vibrational energy levels, and electronic energy
levels. These have been experimentally determined in some cases, but not
all. Ab initio methods can provide rotational constants and vibrational
energy levels. Many of the errors for the integrated heat capacity obtained
from ab initio calculations come from ignoring spin orbit splitting in
open-shell linear molecules or ignoring low lying electronic states and errors in the
vibrational energy levels for non-harmonic vibrations. They can amount to
≈1kJ/mol. The errors in entropies obtained from calculations arise from the
same sources and can amount to ≈10 J/mol K (so @ 300 K ≈3kJ/mol)
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