The two-particle-hole Tamm-Dancoff approximation (2ph-TDA) equations for closed-shell atoms and molecules

An approximation scheme for the one-particle Green's function referred to as the two-particle-hole Tamm-Dancoff approximation (2ph-TDA) is introduced by means of a well defined infinite partial summation of the perturbation expansion for the so-called self-energy part. A spin-free formulation of the working equations is presented for molecular applications. It is demonstrated that the 2ph-TDA is a useful tool for a theoretical treatment of inner valence ionisation processes. A discussion of the physical content and the relationship to other approaches shows the central role of this approximation.