National Institute of Standards and Technology
Computational Chemistry Comparison and Benchmark DataBase
Release 17bSeptember 2015
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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How to get an enthalpy of formation from ab initio calculations

Conversion from 0 K to 298 K

To convert from 0 K to some other temperature we again need to define a reaction. This time the reaction involves the elements in their standard states. For C2H4 we have the reaction:
C2H4 = 2 C(s) + 2 H2(g) We have the enthaply of formation at 0 K of 61.1 kJ/mol for C2H4. (From the HF/6-31G* calculation on the previous page.) Because the products of this reaction are standard states their enthalpy of formation is defined as 0 kJ/mol. We use the integrated heat capacities to convert the reaction energy to 298 K. The integrated heat capacities we obtain from the calculation using standard statistical thermodynamics. (See section I.D. A brief description of the thermochemical quantities and methods.) Section III.A.3 lists entropies and integrated heat capacities for the species and calculations in the CCCBDB and the experimental values for the elements in their standard states are listed in section II.B.

  reactant products reaction total
Species C2H4 2 C(s) 2 H2(g)
Enthalpy of formation at 0 K (kJ/mol) 61.1 0 0 -61.1
Integrated Heat Capacity 0 K to 298 K (kJ/mol) 10.502 2 × 1.050 2 × 8.468 8.534
Enthalpy of formation at 298 K (kJ/mol) 52.6 0 0 -52.6