A posteriori Brillouin—Wigner correction of limited multireference configuration interaction: analysis for an (H₂)₄ cluster model

Abstract

An a posteriori Brillouin—Wigner correction is applied to a limited multireference configuration interaction study of a simple model problem consisting of four H₂ molecules arranged in a distorted octagonal conformation in which the degree of quasidegeneracy depends on a single parameter designated a. Double reference configuration interaction calculations are performed as a function of the parameter a. A posteriori Brillouin—Wigner corrections are applied and compared with a generalization of the Davidson correction for multireference applications. The calculated total energies for the ground state are compared with full configuration interaction studies carried out within the same basis set of Gaussian-type functions. The basis set dependence of the problem is explored by performing calculations in two small Gaussian basis sets: one of minimum basis set quality and the other of ‘double ζ’ type.