PERTURBATIVE CORRECTIONS TO ACCOUNT FOR TRIPLE EXCITATIONS IN CLOSED AND OPEN-SHELL COUPLED-CLUSTER THEORIES

The application of the coupled cluster method restricted to single and double excitations (CCSD) to a non-Hartree-Fock determinantal reference wavefunction is examined, with particular emphasis on the case of spin-restricted open shell reference states. By considering the perturbation expansions of the true and CCSD energies, expressions are derived which give a perturbative correction for the effects of triple excitations which is consistent to fifth order. The resulting Ansatz, denoted CCSD-T, differs from the standard CCSD(T) correction even in the case where Brillouin's theorem holds, and ought to be superior, since it correctly includes more disconnected triples terms up to fifth order. This assertion is investigated by test calculations, which show that both methods deliver accurate results for both closed and open shell molecules.