Valence bond model analysis of homonuclear alkali and halogen diatomics on the basis of exchange perturbation theory

First- and second-order exchange perturbation theory was applied, within the framework of the valence-bond method, to the study of chemical bonding in homonuclear alkali and halogen diatomics. The ultimate aim of the analysis was to derive accurate, yet simple, methods applicable to conformational and reactivity problems in biomolecules. Using a basis set of atomic functions limited to a very few, spherical or polarized, gaussian-type orbitals and two variable parameters with a clear physical interpretation, it was shown that it is possible to reproduce, through a scanning procedure, experimental data on the equilibrium distances, ground-state dissociation energies, and vibrational constants with an accuracy comparable to that obtained with extensive ab initio calculations