Heats of Formation from DFT Calculations:  An Examination of Several Parameterizations

DFT calculations of the total energy of molecules (B3LYP/6-31G**//rhf/6-31G**) can be converted to enthalpies of formation by various new parametrization schemes. Three schemes follow the hierarchy proposed by Benson and Buss, of atom, bond, and group contributions. In addition, a modified atomic scheme was found to give better results than the atomic scheme with fewer parameters than the bond scheme. The lower level parametrizations could fit more compounds but had lower precision. To compare the schemes a standard set of 180 compounds was treated by all methods. The results are:  Method, number of compounds, number of parameters, standard deviation (kcal/mol):  Atom, full:  250, 18, 2.96; std:  180, 12, 2.64; Modified atom, full:  248, 23, 2.53; std:  180, 18, 1.97; Bond, full:  229, 40, 1.90; std:  180, 32, 1.72; Group, full:  183, 79, 1.39; std:  180, 79, 1.26.