Q. Why Rydberg basis sets are necessary in core-electron excitations and ionizations

 

A. Since Rydberg states appear in the energy range similar to the valence states, it is quite natural that Rydberg states appears in the core-electron process as the many-electron processes. For example, in the satellite peaks related to the core-electron ionizations, the electronic structure of the states may involve two electron processes: one-electron core-ionization and one-electron Rydberg excitation.