VALENCE-BOND CALCULATIONS OF THE POTENTIAL-ENERGY SURFACE FOR CH4-]CH3+H

A valence bond study of the potential energy surface for methane CH4-- > CH3 + H is performed at the 6-31G level using (i) a valence bond sel f-constituent field (VB-SCF) method; (ii) a valence bond configuration interaction (VBCI) method; and (iii) an antisymmetrized product of st rong-orthogonal geminals (APSG) method (also in VB form). The calculat ions show that, although the energies are somewhat inferior (on an abs olute scale) to those obtained in very large CI calculations, the VB r educed potential energy surfaces behave better, in the intermediate ra nge 2-3 Angstrom, than those obtained using (i) the Moller-Plesset fou rth-order perturbation (MP4) approximation, (ii) configuration interac tion with all singles and doubles (CISD), and (iii) coupled clusters w ith all singles and doubles (CCSD). The results are in very good agree ment with those obtained from multi-reference configuration interactio n (MR-CISD) calculations. The lower absolute energies obtained in the very extensive CI calculations indicate a better description of electr on correlation, both in the molecule and in its dissociation products, but evidently they do not imply a better overall description of the P E surface. The remarkable fact is that a single VB structure, with car efully optimized orbitals, provides an excellent description of the wh ole dissociation process.