Many-Body Brillouin-Wigner Theories: Development and Prospects

We describe a quantum chemical project focussed on the development of state-specific many-body Brillouin–Wigner methods which was undertaken during the period 1994 to the present day. The Brillouin–Wigner methodology has been shown to provide an approach to the many-body problem which is especially useful when a multireference formulation is required since it completely avoids the ‘intruder state’ problem that often plagues the traditional Rayleigh–Schrodinger expansion. The many-body Brillouin–Wigner approach provides the basis for robust methods which can be applied routinely in situations where the more familiar single-reference formalism is not adequate in Coupled Cluster (cc) and Perturbation Theory (pt) formalisms. It can also be employed in Configuration Interaction (ci) studies. Although the Brillouin–Wigner expansion is not itself a ‘many-body’ theory, it can be subjected to a posteriori adjustments which removes unphysical terms, which in the diagrammatic formalism correspond to unlinked diagrams, and recover a ‘many-body’ method. As well as reviewing progress and prospects of the state-specific Brillouin–Wigner approach, we describe the new methods of communication that were deployed to facilitate effective collaboration between researchers located as geographically distributed sites. A web-based collaborative virtual environment (cve) was designed for research on molecular electronic structure theory which will support the development of quantum chemical methodology for challenging applications. This cve was developed whilst actually carrying out a significant but specific, ‘real life’ quantum chemical project so that those features which were found to be useful in facilitating remote collaboration could be evaluated and incorporated in an emerging framework.