A simple scheme for the direct calculation of ionization potentials with coupled-cluster theory that exploits established excitation energy methods

Vertical ionization potentials can be obtained from existing computer programs for the high-level treatment of excited states by simply including a continuum orbital in the basis set. Exploiting this feature easily allows final state energies for ionized states to be calculated at several previously untested levels of theory that go beyond the equation-of-motion coupled-cluster singles and doubles model. Values obtained for N-2, CO, and F-2 with the most theoretically complete approximations studied here (those based on the CCSDT-3 and CC3 parametrizations of the neutral ground state) are in excellent agreement with experiment when a large basis set is used. (C) 1999 American Institute of Physics. [S0021-9606(99)30143-4].