The band description of materials with localizing orbitals

Density functional theory is a form of many body theory that maps the problem onto an equivalent single particle‐like system by limiting to the ground state (or some limited ensemble). So it should be surprising that this ground state theory could have any relevance whatsoever to the excitation properties of a material, and yet it does when used carefully! However , the most interesting materials involve active orbitals that are at least partially localized in space, and this has profound effects both on the ground state and the excitation spectrum. My long term interest is in Ce and actinide compounds, where the popular concerns are mixed valence, heavy fermions, and the various forms of magnetic transitions. Band structure calculations can give a great deal of information concerning the mechanisms and degree of the localization as will be shown by examples using the Ce and U Ll 2 structured materials and the Ce cubic Laves phase materials. There are some difficulties due to an incomplete knowledge of the functions involved that causes an underestimate of the local character. This will be illustrated and discussed.