ENTHALPIES OF FORMATION FROM THEORETICAL METHODS - A NEW APPROACH BASED ON DENSITY-FUNCTIONAL THEORY PLUS SEMIEMPIRICAL CORRECTIONS

A theoretical method to obtain enthalpies of formation of organic comp ounds is considered. The application of the method requires modest com puter facilities, because it is based, for its most computationally de manding part, on density functional theory (DFT) which is now implemen ted in many efficient programs. The proposed procedure is based on ''i sodesmic reactions''. Starting from the hypothesis that the main sourc e of error in density functional calculations on isodesmic reactions i s the neglect of intra-molecular dispersive contributions to the elect ronic energy, the DFT results are corrected with a simple semiempirica l formula involving a limited number of parameters. The involved appro ximations are checked through MP2 calculations with localized molecula r orbitals, using the orbital invariant formulation of Moller-Plesset perturbation theory proposed by Pulay and Saebo. The method has been t ested on a set of hydrocarbon molecules: the final results agree well with experimental enthalpies of formation, the standard deviation bein g lower than 1.0 kcal mol(-1).