Nonlocal Density Functional Calculation of Gas Phase Heats of Formation

A general procedure is presented for computing the gas phase heats of formation of a wide variety of organic compounds. AE for the formation of the molecule from its elements at 0°K is obtained from density functional calculations (Gaussian 92/DFT) for optimized geometries. This result is converted to AH at 298*K by assuming ideal behavior and adding the translational, rotational and vibrational energies. Additive correction terms corresponding to the various coordination states of the carbons, nitrogens and oxygens were developed using a data base of 54 compounds. The experimental AHf values of these compounds are then reproduced with an average absolute error of 3 kcal/mole and a standard deviation of 4 kcallmole. For a group of ten test cases that were not part of the data base, the average absolute error is 3.5 kcal/mole and the standard deviation is 4.1 kcal/mole.