A second-order correction to singles and doubles coupled-cluster methods based on a perturbative expansion of a similarity-transformed Hamiltonian

We present a new ansنtz for deriving perturbative corrections to single-reference coupled-cluster based methods. The methods are based on a perturbative expansion of the Hamiltonian formed by a similarity transformation of the normal Hamiltonian. These new methods naturally contain triple and quadruple excitations at second order. However, because of a factorization approximation, the cost of the method only scales as n7. Results are presented for a second-order correction to the optimized-orbital coupled-cluster doubles method [OD(2)]. For cases where the standard CCSD(T) method performs poorly, OD(2) gives significantly improved results.