Density Functional Theory (DFT) Study of Enthalpy of Formation. 1. Consistency of DFT Energies and Atom Equivalents for Converting DFT Energies into Enthalpies of Formation

A simple atom equivalent method for converting density functional theory (DFT) energies to enthalpies of formation is described and its performance, in conjunction with six DFT methods, was examined. For 23 stable hydrocarbons with well-established experimental data, the root mean square deviations between the calculated and experimental enthalpies of formation range from 1 to 6 kcal/mol. The smallest deviation was obtained with the B3LYP energies, and the largest deviation was obtained with the LSDA energies. The B3LYP atom equivalents of carbon and hydrogen derived from stable hydrocarbons were used without adjustment to calculate the enthalpies of formation of some free radicals and carbocations. The mean deviation between the calculated and experimental results is about 2 kcal/mol, which is of the same order as experimental uncertainties for these highly reactive species.