Correlation energy contributions to reaction heats

New, more accurate, Hartree‐Fock limit energies ( E HF ) for ethane and ethylene are obtained from SCF total molecular energie using Ermler and Kern's procedure. These results, together with E HF values for other small closed shell molecules, are employed to calculate correlation energy ( E c ) contributions to reaction heats. Cancellation to within 98% of the total E c involved, and often to more than 99%, is found for a wide variety of chemical reactions, which strongly suggests that there are systematic regularities in the contribution to E c from the different kinds of electron pairs in the valence shell. Assuming trictly localized pairs occupying orbitals having strongly directional character, E c for the valence shell is evaluated in terms of E c per lone pair, E c per XH bond, and E c per X/X shared pair for Ne and for molecules containing first row atoms, where X is C, N, O, and F.