Accidental Degeneracy in the Energy Bands of Crystals

TLDR: In this paper, the authors investigated the circumstances under which two wave functions occurring in the Hartree or Fock solution for a crystal can have the same reduced wave vector and the same energy.

Abstract: The circumstances are investigated under which two wave functions occurring in the Hartree or Fock solution for a crystal can have the same reduced wave vector and the same energy. It is found that coincidence of the energies of wave functions with the same symmetry properties, as well as those with different symmetries, is often to be expected. Some qualitative features are derived of the way in which energy varies with wave vector near wave vectors for which degeneracy occurs. All these results, like those of the preceding paper, should be applicable also to the frequency spectrum of the normal modes of vibration of a crystal.