Use of Brueckner orbitals in second-order approximate coupled-cluster with singles and doubles (CC2) model

Brueckner orbitals are introduced into the CC2 (second-order approximate coupled-cluster with singles and doubles) model. The computational cost of the Brueckner CC2 model is virtually identical to the conventional CC2 model. Brueckner CC2 is tested for the ground-state energies of several well-defined molecules as well as problematic ones. Significant improvement compared to the conventional CC2 model is observed when large orbital relaxation is invoked. Evaluation of analytical gradients and excited states is also discussed.