Correlation-consistent valence bond method with purely local orbitals. Application to H₂, Li₂, FH, F₂, and collinear H₃ and Li₃

We propose a general method for constructing ab initio valence bond wave functions. The emphasis is put on building compact wave functions designed to be as close as possible to the concept of chemical bonding schemes. This is achieved through the use of strictly local fragment orbitals, leading to nonorthogonal configuration interaction. A set of rules is proposed for selecting the configuration list such that correlation consistency is enforced over the potential surface. The compactness of the wave function is ensured by correlating only the electrons occupying active orbitals, defined as the orbitals directly involved in bond breaking or bond making. The method is applied to the dissociation of H₂, Li₂, FH, and F₂, and to the collinear exchange reactions X + X₂ → X₂ + X (X = H, Li). The dimensions of the corresponding valence bond CIs are respectively 6, 12, 31, and 24 symmetry-adapted configurations for the dimers, and 26 and 60 for the trimers. All calculated equilibrium bond lengths, dissociation energies, and reaction barriers are found to agree, within 0.02 Å and 3 kcal/mol, with best reference calculations in the same basis set. The method appears to be well suited for the calculation of diabatic curve crossing diagrams as introduced by Shaik and Pross.