Use of effective core potentials in perturbation corrections to the Koopmans theorem: Vertical ionization potentials of Cl2, ClN3, and ClNCO

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Abstract
The valence‐shell vertical ionization potentials of Cl2 were calculated by perturbation corrections to the Koopmans theorem using a traditional effective core potential based on a Phillips–Kleinman derivation and an improved effective core potential obtained by Christiansen, Lee, and Pitzer. Comparison of the results with an all‐electron calculation demonstrated the reliability of the Christiansen–Lee–Pitzer effective core potential, which was then used to compute the vertical ionization potentials of ClN3 and ClNCO. The results shed new light in the interpretation of the photoelectron spectra of these molecules.