Nonsymmetrical perturbation theory for improving coupled-cluster wave functions

A general nonsymmetrical formulation of multireference perturbation theory (PT) is used to correct the energy and wave function obtained from limited coupled-cluster (CC) calculations. Following the spirit of CC theory, we use the Hartree-Fock wave function in bras while the CC wave function is used as the ket reference. The resulting PT is compared to the symmetrical (Hermitean) formulation and to the systematic CC(n)PT(m) theory of Hirata et al. Numerical results up to the fifth order in PT are obtained in a determinantal formalism for a few sample molecules. The effect of partitioning is also studied. (C) 2002 Wiley Periodicals, Inc.