Size-extensivity correction for the state-specific multireference Brillouin-Wigner coupled-cluster theory

We present a simple a posteriori correction for the state-specific multireference Brillouin-Wigner coupled-cluster (MR BWCCSD) theory, which eliminates its size-extensivity error. In the converged amplitudes we drop terms that were identified to be responsible for the lack of size extensivity. We performed MR BWCCSD calculations with this correction on CH(2), SiH(2), twisted ethylene, F(2), and ozone that are all, from the computational point of view, typical representatives of two-reference problems. Comparison with rigorously size-extensive calculations and experiment shows that the size-extensivity error of the corrected MR BWCCSD is only a few tenths of kcal/mol. (C) 2000 American Institute of Physics. [S0021-9606(00)30315-4].