An intermediate state representation approach to K-shell ionization in molecules. I. Theory

The general intermediate state representation (ISR) for single-electron ion ization is adapted to the case of K-shell (or core-level) ionization in mol ecules. The development is based on the so-called core-valence separation ( CVS) approximation leading to a considerable simplification of the ISR secu lar equations. Using the CVS approximation the core-level ISR can be formul ated entirely in terms of the intermediate states of the valence electron e xcitation problem, which allows one to construct consistent nth-order appro ximation schemes for the (single-hole) ionization energies by a specific ex tension of the (n-2)-nd order ISR approximation for electronic excitation. In particular, the CVS-ISR concept is used to derive a consistent fourth-or der approximation for core-level ionization based on the existing second-or der algebraic-diagrammatic construction [ADC(2)] approximation to electron excitation. The computational scheme combines the diagonalization of a Herm itian secular matrix with finite perturbation expansions for the secular ma trix elements. The explicit configuration space is spanned by one-hole (1h) , two-hole-one-particle (2h-1p), and (3h-2p) ionic states with exactly one hole in the core-level shell of interest, while the configurations consider ed implicitly via perturbation theory extend to the class of 5h-4p states. A characteristic of the method is that the dominant valence electron relaxa tion effect is accounted for at the post-Hartree-Fock (HF) level. This call s for the relatively high order of perturbation-theoretical consistency, bu t avoids, on the other hand, the necessity of a localized (symmetry breakin g) one-particle representation in the case of molecules with equivalent 1s orbitals. The method is size consistent and thus suitable for applications to large systems. (C) 2001 American Institute of Physics.