Generalized self-consistent valence bond method for ground and excited potential energy surfaces

Abstract

A new method is presented for computing ground and excited states potential energy surfaces, within the context of the valence bond (VB) theory, retaining the possibility of describing electronic molecular changes in terms of valence bond structure concepts. The separated electron pair (SEP) theory in the rank two geminal approximation is reformulated and compounded with the straightforward VB formalism: the set of non-orthogonal orbitals generated by the SEP optimization is used to construct VB structures, which allow a compact and correct description of the wavefunction. The procedure is applied to the study of the ground and excited states of LiH of Σ and Π symmetry (A 1Σ+, C 1Σ+, a 3Σ, b 3Π, B 1Π). All the qualitative features of these states are reproduced, including the peculiarities of the A 1Σ+ state. Each state is described by one to four structures, from small values of the internuclear distance up to dissociation: a clear interpretation of the wavefunction is therefore provided.