Applications of open-shell coupled cluster theory using eigenvalue-independent partitioning technique: approximate inclusion of triples in IP calculations

Abstract

Using our eigenvalue-independent partitioning (EIP) approach for the calculation of open-shell coupled cluster (CC) energy differences, we have computed the ionization potentials of HF and H₂O using basis sets with and without polarization functions. Our results include the three-body cluster operator for the ionized states at the lowest order of approximation. It is found that a CCSD calculation for the ground state, followed by a CCSD calculation for the ionized states - with additional inclusion of triples using the converged CCSD amplitudes - produces results that are accurate up to third order and recovers the relaxation and differential correlation energies consequent on ionization in a balanced and compact manner.