Examination of the limits of accuracy of the extended Koopmans' theorem ionization potentials into excited states of ions of liH, He 2 , and Li 2

A comparison is made between the ionization potentials ( IPS ) calculated by the extended Koopmans' theorem ( EKT ) and by taking energy differences (Δ CI ) from configuration interaction calculations in the same basis. Several ionization potentials were calculated for LiH, He 2 , and Li 2 . The best Δ IP , the difference between the EKT IP and the corresponding Δ CI value, was 0.05 meV for the 2σ orbital for LiH and 83.5 meV for the 3σ orbital. The Δ IPS for He 2 were 0.7 meV for the 1σ u orbital, 6 eV for the 2σ u orbital, 5 meV for the 2σ g orbital, and 3 eV for the 3σ g orbital. The Δ IPS for Li 2 are 0.1 meV for 2σ g , 53 meV for 3σ g , 0.6 meV for 2σ u , and 1.73 eV for 3σ u . © 1994 John Wiley & Sons, Inc.