Multireference Brillouin-Wigner coupled cluster (MR-BWCC) theory applied to the H8 model: Comparison with CCSD(T) theory

Multireference Brillouin-Wigner coupled cluster theory is applied to the H8 model, a model in which the degree of quasidegeneracy is a function of a single geometrical parameter a. Using a 6-31G basis set, multireference Brillouin-Wigner coupled cluster theory (MR-BWCC) is applied to the H8 model as a function of a both with and without an a posteriori correction for the lack of extensivity. The resulting potential energy curve is compared with the corresponding curve obtained by the application of single reference CCSD theory (coupled cluster singles and doubles), and with the curve obtained after adding a perturbative estimate of the triple excitation energy component, CCSD(T). The MR-BWCCSD, CCSD, and CCSD(T) curves are also compared with the full configuration interaction (FCI) curve defining the exact solution within the chosen basis set. (c) 2005 Wiley Periodicals, Inc.