Classical valence bond approach by modern methods

Valence bond (VB) theory expresses the molecular wave function as a combination of explicit covalent and ionic structures based on pure atomic orbitals (AOs) or hybrid atomic orbitals (HAOs). The resurgence of modern classical VB theory involves the development of several methodological advances which allowed new and more accurate applications of the theory. The basic method, which was devised by Balint-Kurti and van Lenthe, is called the valence bond self-consistent field (VBSCF) method. The method optimizes VB orbitals and structural coefficients simultaneously and uses the same set of HAOs for all the structures. The VBSCF method branches into two sets of methods, one is the breathing-orbital VB (BOVB) method, where one uses the same VBSCF wave function. The valence bond second-order perturbation method (VBPT2) uses perturbation theory, taking the VBSCF wave function as the zeroth-order reference. The VBSCF method permits complete flexibility in the definition of the orbitals used for constructing VB structures.