Multireference Brillouin-Wigner coupled clusters method with noniterative perturbative connected triples

We developed and implemented the state-specific Brillouin-Wigner coupled cluster method with singles, doubles, and noniterative perturbative triples, called MR BWCCSD(T), for a general number of closed- and open-shell reference configurations. To assess the accuracy of the method, we performed calculations of the three lowest electronic states of the oxygen molecule and of the automerization barrier of cyclobutadiene. For the oxygen molecule, the results were in a good agreement in comparison with those of the iterative MR BWCCSDT alpha method. For cyclobutadiene, the effect of connected triples was found to be minor, which is in agreement with the previous study by and Balkova and Bartlett [J. Chem. Phys. 101, 8972 (1994)]. (c) 2006 American Institute of Physics.