An efficient Fock space multi-reference coupled cluster method based on natural orbitals: Theory, implementation, and benchmark

We present a natural orbital-based implementation of the intermediate Hamiltonian Fock space coupled-cluster method for (1, 1) sector space. The use orbitals significantly reduces computational cost and can automatically choose an appropriate set active orbitals. new retains charge transfer separability original gives excellent performance valence, Rydberg, charge-transfer excited states. It offers significant advantages over popular equation motion coupled cluster states dominated by single excitations.