Perturbative triples corrections in state-specific multireference coupled cluster theory.

We formulated and implemented a perturbative triples correction for the state-specific multireference coupled cluster approach with singles and doubles suggested by Mukherjee and co-workers, Mk-MRCCSD [Mol. Phys. 94, 157 (1998)]. Our derivation of the energy correction [Mk-MRCCSD(T)] is based on a constrained search for stationary points of the Mk-MRCC energy functional together with a perturbative expansion with respect to the appearing triples cluster operator. The Lambda-Mk-MRCCSD(T) approach derived in this way consists in (1) a correction to the off-diagonal matrix elements of the effective Hamiltonian which is unique to coupled cluster methods based on the Jeziorski-Monkhorst ansatz, and (2) an asymmetric energy correction to the diagonal elements of the effective Hamiltonian. The Mk-MRCCSD(T) correction is obtained from the Lambda-Mk-MRCCSD(T) method by approximating the singles and doubles Lagrange multipliers with the corresponding cluster amplitudes. We investigate the performance of the Mk-MRCCSD(T) method by applying it to the potential energy curve of the BeH(2) model and F(2) and the geometry and harmonic vibrational frequencies of ozone. Computation of the energy difference between the mono- and bicyclic forms of the 2,6-pyridyne diradical illustrates the potential of Mk-MRCCSD(T) as a tool for the study of realistic chemical problems requiring multireference zeroth-order wave functions.