AN ALTERNATIVE EXPRESSION FOR THE PERTURBATIVE TRIPLES CORRECTION TO BRUECKNER COUPLED-CLUSTER DOUBLES THEORY

The Brueckner model, which is a version of coupled-cluster theory where orbitals are optimized to yield zero single-excitation amplitudes (T1=0), naturally leads to the definition of an effective one-particle Hamiltonian F (or Brueckner Hamiltonian) which effectively accounts for correlation effects in an independent particle type model. We propose to use the eigenvalues of the F-matrix as an alternative approach to the perturbative triples corrections (T) of Brueckner doubles (BD) theory. The new method is tested in a highly demanding case, the ozone molecule with bond-lengths symmetrically stretched up to 1.3 R(e). When compared with results obtained at the full singles, doubles, and triples coupled cluster (CCSDT), and multi-reference configuration interaction levels of theory, the new BD(T) model gives better quantitative agreement for total energies than the previously proposed approach.