On perturbation expansions of the extended Koopmansʹ theorem

Ionization potentials obtained as energy differences and using the extended Koopmansʹ theorem expression, respectively, have been calculated by analytic perturbation theory through 12-th order. A comparison of the expansions shows that the extended Koopmansʹ theorem for the lowest ionization potential is exact for each order in the perturbation expansion.