FOCK-SPACE MULTIREFERENCE COUPLED-CLUSTER THEORY - 4TH-ORDER CORRECTIONS TO THE IONIZATION-POTENTIAL

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