Electron correlation: Keeping close to an orbital description

This article deals with excited‐state properties and spectroscopies which probe them. We argue that the orbitals obtained in local density theory form an excellent basis for discussing excited states and that the self‐energy operator (or optical potential) associated with one‐electron‐type excited states is closely diagonal in such a basis. This diagonality often leads to considerable simplifications. We also discuss the reasons for the successes of the “GW‐approximation” for the optical potential in predicting excited states. We further discuss a recent approach to calculate spectroscopic data. In particular, we present explicit results of this approach to photoelectron emission. The approach treats correlations within the target exactly but neglects some terms in the coupling between target and probe electron (= the photoemitted electron). This leads to closed expressions which exhibit a number of qualitative features. One is the cancellation of intrinsic and extrinsic losses at threshold excitation. Another is the broadening of the quasi‐particles; we have energy‐broadening for the occupied states but broadening through decaying wave functions for unoccupied states.