Hartree-Fock, Hartree-Fock-plus-correlation and self-consistent Kohn-Sham calculations are performed on a set of hydrocarbon isodesmic reactions, i.e. reactions among hydrocarbons in which the number and type of carbon-carbon and carbon-hydrogen bonds is conserved. It is found that neither Hartree-Fock nor Kohn-Sham methods correctly predict standard enthalpies, DELTAH(r)(298 K), of these reactions, even though - for reactions involving molecules containing strained double bonds - the agreement between the theoretical estimates and the experimental values of DELTAH(r) seems to be improved by the self-consistent solution of the Kohn-Sham equations. The remaining discrepancies are attributed to intramolecular dispersion effects, that are not described by ordinary exchange-correlation functionals, and are eliminated by introducing corrections based on a simple semi-empirical model.
