ON PERTURBATION EXPANSIONS OF THE EXTENDED KOOPMANS THEOREM

Ionization potentials obtained as energy differences and using the ext ended Koopmans' theorem expression, respectively, have been calculated by analytic perturbation theory through 12-th order. A comparison of the expansions shows that the extended Koopmans' theorem for the lowes t ionization potential is exact for each order in the perturbation exp ansion. (C) 1998 Elsevier Science B.V. All rights reserved.