A STUDY OF THE C-LI BOND IN METHYLLITHIUM AND OF THE POTENTIAL-ENERGYSURFACE FOR BOND BREAKING

An ab initio valence bond study of the potential energy surface for me thyllithium, with special reference to dissociation by symmetric stret ch of the C-LI bond, has been performed and compared with previously p ublished results. New calculations using many-body perturbation theory (MBPT at the MP4 level) and coupled-cluster methods with all singles and doubles (CCSD) have also been made for comparison purposes. The VB description of the bond in terms of one covalent and two ionic struct ures is found to be highly satisfactory throughout the whole dissociat ion process: the ionic character of the bond is about 38% at equilibri um, going smoothly to zero above about 4 Angstrom, while the energy cu rves are competitive with the MP4 and CCSD curves and appear to give a n excellent general description of the bond-breaking process.