Broken Orbital-Symmetry and the Description of Hole States in the Tetrahedral [CrO4]= Anion. I. Introductory Considerations and Calculations on Oxygen 1s Hole States

The localization of holes in systems containing spatially equivalent sites is discussed in terms of a simple one-particle model in which quantum mechanical delocalization effects compete with essentially classical polarization or dielectric relaxation effects. The predictions of the model for a tetrahedral system like CrO4= compare favourably with the results of symmetry unrestricted SCF calculations on O1s hole states. The connection with a CI treatment using symmetry-restricted MOs is discussed. The calculated ionization energies are finally compared with XPS measurements on Na2CrO4. To this and the crystal surrounding of the CrO4= anion has been represented by a point charge model and the ensuing Madelung field was included in the SCF calculations. In contrast to the Td restricted result of 551.4 eV, the completely localized C3v result of 532.6 eV is in satisfactory agreement with the experimental data which are found around 530.0 eV.