CUMULANT APPROACH AND COUPLED-CLUSTER METHOD FOR MANY-PARTICLE SYSTEMS

Cumulants represent a natural language for expressing macroscopic prop erties of many-body systems. The most important property of cumulants is that of size consistency, i.e. a cumulant expression for an extensi ve variable scales with the size of the system, independent of possibl e further approximations used in the evaluation procedure. Cumulants c an be considered as a generalization of linked diagrams known from dia grammatic technique of many-body theory. In this paper we outline a re cently introduced method based on cumulants in order to derive express ions for zero-temperature properties of many-particle systems, i.e. th e ground-state energy, static expectation values and dynamical correla tion functions. This cumulant formalism allows one to describe weakly and strongly correlated systems along the same lines. We show that the coupled-cluster method known from quantum chemistry can be derived fr om our cumulant approach. Finally, we demonstrate the usefulness of th e cumulant method by applying it to examples from solid-state physics and quantum chemistry.