N. Vaval et al., FOCK-SPACE MULTIREFERENCE COUPLED-CLUSTER THEORY - 4TH-ORDER CORRECTIONS TO THE IONIZATION-POTENTIAL, Chemical physics letters, 209(3), 1993, pp. 292-298
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.