Abstract
Green's function studies of the valence-shell ionization spectra of N 2 and CO are reported. The employed method of calculation is the previously derived extended two-particle-hole Tamm-Dancoff approximation (2ph-TDA). This approximation gives a consistent treatment of the one-particle Green's function through third order in the electronic repulsion. Very accurate results are obtained for the outer-valence ionization potentials. The extended 2ph-TDA also allows for an improved description of the inner-valence ionization where as a rule the single-particle picture of ionization no longer applies. The present calculations are devised as a model application of the extended 2ph-TDA. A large orbital basis set has been employed and only minor technical approximations have been introduced. Comparison is made with results from other theoretical approaches, in particular with those of the related third-order equations-of-motion (EOM) scheme.
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