Two new classes of non-iterative coupled-cluster methods derived from the method of moments of coupled-cluster equations

Abstract

Two recently proposed classes of non-iterative coupled-cluster (CC) methods derived from the method of moments of CC equations (MMCC) are discussed. The first approach, termed MMCC/PT, combines the MMCC formalism with a simplified form of multi-reference perturbation theory. The second approach, which leads to completely renormalized (CR) CC methods employing the left eigenstates of the similarity-transformed Hamiltonian, such as CR-CCSD, exploits the recently developed biorthogonal formulation of the MMCC theory. Both approaches are capable of improving the results of standard CC and equation-of-motion CC (EOMCC) calculations for ground-state potential energy surfaces along bond breaking coordinates and excited states dominated by two-electron transitions with computer costs similar to those characterizing the popular (and failing) CCSD(T) approximation. The performance of the basic MMCC/PT and CR-CCSD approximations, in which non-iterative corrections due to triple excitations are added to the ground-state energies obtained with the CC singles and doubles (CCSD) approach, is illustrated by the results of calculations for the HF, F2 and H2O molecules. The improvements offered by these approaches in the excited-state calculations are illustrated by the results for the vertical excitation energies of the CH+ ion.