Multireference Brillouin-Wigner methods for many-body systems

Brillouin-Wigner perturbation theory has a number of advantages over the much more widely used Rayleigh-Schrodinger perturbation theory in studies of the electron correlation problem in atoms and molecules. This article briefly reviews some historical background, the perceived advantages and disadvantages of the Brillouin-Wigner perturbation theory. The convergence difficulties of the Rayleigh-Schrtkiinger perturbation theory when applied to problems demanding the use of multireference formalism is described. The problems created by the appearance of intruder states, of both the "physical" and the "backdoor" variety, in the multireference Rayleigh-Schrodinger formalism are outlined. Recent progress in the formulation and application of Brillouin-Wigner methods to the atomic and molecular many-body problem is then described. The use of Brillouin-Wigner theory in solving the equations associated with many-body theories, such as coupled cluster theory, is described as well as in the development of a posteriori corrections to theories which contain terms which scale non-linearly with particle number, such as limited configuration interaction or finite-order Brillouin-Wigner perturbation theory. Particular attention is given to multireference Brillouin-Wigner methods. (C) 2001 by Academic Press