A simple CI model for the valence CI bands in CO+2 and isovalent ions. Ionization potentials of CO2 in the 20–30 eV range

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Abstract
The qualitatively correct Mulliken–Walsh Aufbau rules are used to derive a simple CI model for the energy differences between the valence CI bands and the valence MO bands in CO+2 and isovalent ions. The essence of the model is that the valence CI bands form multiplet terms, and at least one member of the multiplet has the same symmetry as, and interacts weakly with, at least one MO ion level. In this case, a common set of orbitals may be derived and modest CI calculations simultaneously provide accurate term splittings and at least one accurate energy separation between one term and one known lower energy MO ion state. CI calculations which include roughly 2000 or fewer spin and space adapted configurations are used to determine all valence CI band ionization potentials of CO2 in the 20–30 eV range. Agreement with observed peaks is typically very good.