Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which belongs to a broad family of state-selective multireference coupled-cluster methods, has been implemented in the ACES II program package at the CCSD level of approximation. The method represents a new approach to quasidegenerate problems, which is able to continuously switch between the single-reference CC in a nondegenerate situation and the Hilbert-space MRCC in a degenerate case. An assessment of the method has been carried out by means of a comparison with the full configuration interaction (CI) treatments of CH2, SiH2, and twisted ethylene diradicals. The problem of size-extensivity is discussed.

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 CH2, SiH2, twisted ethylene, F2, 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.

Automerization reaction of cyclobutadiene and its barrier height: An ab initio benchmark multireference averagequadratic coupled cluster study J. Chem. Phys. 125, 064310 (2006); 10.1063/1.2222366 Continuous transition between Brillouin-Wigner and Rayleigh-Schrödinger perturbation theory, generalized Bloch equation, and Hilbert space multireference coupled cluster Hilbert-space state-universal multireference coupled-cluster ͑MR CC͒ data on cyclobutadiene ͓A. Balková and R. J. Bartlett, J. Chem. Phys. 101, 8972 ͑1994͔͒ were used as a benchmark for testing our recently developed state-specific ͑single-root͒ multireference Brillouin-Wigner coupled-cluster ͑MR BWCC͒ theory. For the energy of activation in the automerization of cyclobutadiene ͑i.e., the energy difference between the square and rectangular structures͒ at the CCSD/͓3s2 p1d/2s͔ level of theory, our MR BWCCSD method gives the value of 6.2 kcal/mol, compared to 6.5 kcal/mol given by MR CCSD. With the cc-pVDZ and cc-pVTZ/cc-pVDZ basis sets, the MR BWCCSD activation barrier is 6.4 and 7.0 kcal/mol, respectively. The effect of the triple excitations ͓in MR CCSD͑T͔͒ and of the frozen core approximation were estimated previously to be below 0.1 kcal/mol and in the opposite direction. This shows the way of how to arrive at a more accurate automerization barrier in future calculations: extension of the basis set seems to be more important than going beyond the CCSD͑T͒ or MB BWCCSD level of the theory.

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