Valence bond perturbation theory for electron correlation

Abstract

A second-order valence bond perturbation treatment (VB–PT) of electron correlation which utilizes the self-consistent valence bond wavefunction as an initial approximation is described. In trial calculations on H2 and LiH, at the equilibrium intranuclear distance, the accuracy is comparable to third-order Hartree–Fock perturbation theory (HF–PT). However, the computation time for VB–PT increases only as NK4 (N is the number of electrons; K the number of basis functions) whereas HF–PT, and other treatments of similar accuracy, are N2K4 procedures. Other advantages of VB–PT include a correct qualitative description of bond dissociation in zeroth-order; an exact breakdown of the perturbation energy into a sum over pairs; potentially weak mixed pair correlations; and the possibility of atoms-in-molecules methods to do away with the basis set problem.