On the adequacy of the molecular-orbital picture for describing ionization processes

General arguments concerning the adequacy of the molecular-orbital (MO ) ionization picture are given. Whereas the MO ionization picture is found to be valid for both outer valence and core electrons, it may completely break down in the inner valence region. The general considerations are supported by many-body calculations on methane, acetylene, hydrogen cyanide, and dinitrogen tetroxide. It is demonstrated that the breakdown of the MO ionization picture is a common phenomenon.