Fock-space multireference coupled-cluster theory. Fourth-order corrections to the ionization potential

Abstract

We propose a highly correlated scheme for implementing a Fock-space multireference coupled-cluster theory for computing directly ionization potentials (IPs). We include all the single and double excitation cluster amplitudes and certain contributions from the triples which contributed at third and fourth orders of perturbation. The computed IP values from our scheme are correct at least up to fourth order. Small numerical test cases are given as examples to illustrate the importance of the fourth-order triples.