Perturbative triples correction for explicitly correlated Mukherjee’s state-specific coupled cluster method

This paper reports incorporation of the perturbative triples correction within the explicitly correlated Mukherjee’s multireference coupled cluster method using the SP ansatz. In accord with the standard approximation, these corrections are not directly entered by the correlation factor amplitudes, but the explicitly correlated part of the effective Hamiltonian is included in full. The performance of the new method is tested on singlet methylene, potential curve of fluorine molecule and automerisation barrier of cyclobutadiene. It has been found that the convergence pattern of the MkCCSD(T)-F12 results with increasing basis set is improved by approximately one cardinal number, as compared to conventional MkCCSD(T). This improvement appears at the level of single and double excitations, whereas no significant impact of the explicit treatment of the electron correlation on the (T) correction has been observed, in analogy to a single-reference approach.