On the calculation of inner shell ionization energies and E.S.C.A. shifts

In the calculation of inner-shell ionization potentials, the Koopmans theorem is unsatisfactory owing to a large ‘relaxation’ of the valence electron distribution, following the creation of an inner-shell ‘hole’. The effects of this relaxation are studied, using a group-function formulation to analyse the energy into core and valence contributions. Applications to the lithium atom and the methane molecule indicate that relaxation effects can be large. The importance of these results in the interpretation of E.S.C.A. data is discussed.