National Institute of Standards and Technology
Computational Chemistry Comparison and Benchmark DataBase
NIST Standard Reference Database 101
IIExperimental data
IIICalculated data
IVData comparisons
VCost comparisons
VIInput and output files
VIITutorials and Units
VIIILinks to other sites
XOlder CCCBDB versions
XIII Vibrations
XIVReaction data
XVEntropy data
XVIBibliographic data
XVIIIon data
XVIIIBad calculations
XIXIndex of properties
XXH-bond dimers

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What's a hartree?

A hartree is a unit of energy used in molecular orbital calculations. A hartree is equal to 2625.5 kJ/mol, 627.5 kcal/mol, 27.211 eV, and 219474.6 cm-1.

The energy reported from a calculation is the energy for the separation of the molecule to nuclei and electrons. For example the HF/6-31G* energy for CH4 is -40.195172 hartrees, which refers to the energy for the reaction CH4 => C6+ + 4 H+ + 10 e-

For semiempirical calculations (AM1, PM3) the energy is reported in hatrees, but the energy refers to the heat of formation for that species at 298 K. For example the PM3 energy for CH4 is -0.020766 hartrees which can be converted directly to a heat of formation of (-0.020766 hartrees)(2625.5 kJ/mol/hartree) = -54.5 kJ/mol.

More on units and conversions can be found near the end of the page discussing thermochemistry (Section I.D.2)