An ab initio form of classical valence-bond theory

In this work the classical valence-bond (VB) theory of the 1930s is recast in a fully ab initio modem form. The basic premises are simply (i) that val ency is associated with singly occupied orbitals on the constituent atoms o f a molecule, more tightly bound electrons being assigned to a "core," (ii) that the valence electrons may be well described using resonance mixtures of alternative spin-paired structures; and (iii) that the structures used a re not confined to a single orbital configuration but may include those of "ionic" type (in which some orbitals are doubly occupied and others are emp ty) and indeed spin-coupled functions based on arbitrary selections of nono rthogonal orbitals. The resultant multiconfiguration VB theory can thus acc ommodate all the basic pictorial concepts introduced by Pauling, Wheland, a nd others in the early days of quantum chemistry. The approach is mathemati cally simple, here being derived from first principles, and permits not onl y the optimization of structure coefficients but also that of the forms of both core and valence orbitals. An illustrative example, in which the benze ne and pyridine molecules are discussed, along with their positive and nega tive ions, confirms the generality and feasibility of the approach. (C) 199 9 John Wiley & Sons, Inc.