Efficient realization of the Fock-space coupled-cluster method with connected triple excitations

A novel approximate variant of the Fock-space multireference coupled-cluster method with inclusion of the triple excitations has been formulated and tested. A starting point for the new scheme is a recently developed intermediate Hamiltonian variant of the FS-CC approach with full inclusion of triples (FS-CCSDT). The essence of the new approach, termed, FS-CCSDT ′ , relies on combining a rigorous treatment of the FS step (scaling as n 7 ) with the ground state CC wave function obtained at the approximate CCSDT-3 level (also scaling as n 7 ). The average deviations for excitation energies of the FS-CCSDT ′ results from the reference values (FCI or experiment) are very close to those of FS-CCSDT.