COMPACT AND ACCURATE VALENCE BOND FUNCTIONS WITH DIFFERENT ORBITALS FOR DIFFERENT CONFIGURATIONS - APPLICATION TO THE 2-CONFIGURATION DESCRIPTION OF F2

A way of computing reliable and very compact ab initio classical valence bond wavefunctions is presented. The method deals with a minimal number of configuration-state functions (CSFs), with only one per Lewis structure necessary to describe the active part of a chemical system since each CSF is allowed to have its specific set of orbitals, different from one CSF to the other. The coefficients of the CSFs and their orbitals, which remain purely local without any delocalization tails, are simultaneously optimized through a non-orthogonal MCSCF technique. The method is applied to the electronic structure of F2. The wavefunction involves only two configurations, and yields a dissociation energy of 28.6 kcal/mol, which compares very well with the estimated full-CI result of 29-30 kcal/mol in an analogous basis set.