Journal of Molecular Structure (Theochem), 276 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP (1992) 159-166 Elsevier Science Publishers B.V., Amsterdam 159 Valence bond model analysis of homonuclear alkali and halogen diatomics on the basis of exchange perturbation theory Giovanni Dotelli”, Erminio Lombardi” and Laurens Jansenb “Dipartimento di Chimica Zndustriale e Zngegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan (Italy) ‘c/o Theoretische Physik, ETH-Hiinggerberg, 8093 Ziirich (Switzerland) (Received 27 January 1992) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Abstract 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. INTRODUCTION The quantitative understanding of molecular stability is a key issue in chemistry and molecular physics, and a large variety of theoretical methods is dedicated to this problem. Among such methods, limiting our- selves to the most recent developments and without claiming completeness, are: the ab initio molecular theory of Pople et al. [1,2] (GAUSSIAN 1 (Gl) and GAUSSIAN z (G2) methods); the multi-structure valence bond method [3]; the application of density-functional theory (DFT) to molecular systems [4]; the Green’s function Monte Carlo (GFMC) methods [5] applied to up to lo- electron systems; ab initio pseudopotential approaches [&9] which are used extensively for studying the Cs, molecule and other alkali diatomics, the alkaline earth hydrides and the diatomic halogen molecules; the multicon- Correspondence to: E. Lombardi, Dipartimento di Chimica Industriale e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32,20133 Milan, Italy. 0166-1280/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.