EXCITATIONS IN A GENERALIZED DENSITY FUNCTIONAL THEORY

It is shown that the Kohn-Sham version of density functional (DF) theory can be reformulated such that certain critical points of the original derivation are avoided. Moreover, the new foundation of DF theory proves to apply to any eigenstate of N interacting electrons moving in the Coulombic potential of atomic nuclei or any external potential. Naive applications of DF theory to electronic excitations of atoms and molecules that are based on DELTA-SCF calculations, for example, X-ray absorption energies, may therefore be considered legitimate. The total energy differences associated, for example, with photoemission from extended solids are shown to consist of two contributions: the respective energy band difference and a correction that can be of the same order of magnitude. Hence, the familiar statement that the Kohn-Sham band structure of semiconductors and insulators yields the 'wrong gap energy" is physically meaningless.