Simple procedure for calculations of relaxation and correlation corrections to Koopmans’ theorem

We proposed a simple procedure for calculating the ionization energy which to some extent takes into account the relaxation and correlation effects. The procedure is based on the perturbation treatments of both the closed-shell ground state the open-shell ionized state. In order to obtain the unperturbed wavefunction.for the ionized state, we present a simple method. The relation among the Koopman’s theorem. Δ SCF-like method- and the purturbation theory is discussed.