Towards the multireference Brillouin-Wigner coupled-clusters method with iterative connected triples:: MR BWCCSDT-α approximation -: art. no. 181101

We developed and implemented an approximation of the state-specific Brillouin-Wigner coupled-cluster method with singles, doubles, and triples, called MRBWCCSDT-alpha, for a general number of closed- and open-shell reference configurations. The accuracy of the method is assessed on the calculation of the oxygen molecule in the X-3 Sigma(-)(g), a(1)Delta(g), and b(1)Sigma(+)(g) states and the results of this multireference treatment are compared with previous MRBWCCSD results and with those obtained by the doubly ionized similarity transformed equation-of-motion CCSD and multireference configuration interaction methods and with experimental spectroscopic data. Explicit tests of the size-extensivity of the MRBWCCSDT-alpha method with iterative size-extensivity correction are also performed. (c) 2005 American Institute of Physics.