Interstitial electronic localization.

We investigate the ground-state properties of a collection of N noninteracting electrons in a macroscopic volume Omega also containing a crystalline array of N spheres of radius rc, each taken as largely impenetrable to electrons and with proximity of neighboring excluding regions playing a key physical role. The sole parameter of this quantum system is the ratio rc/rs, where rs is the Wigner-Seitz radius. Two lattices (fcc and bcc) are selected to illustrate the behavior of the system as a function of rc/rs. As this ratio increases valence electrons localize in the interstitial regions and the relative bandwidth epsilonF/epsilonF0 is found to decrease monotonically for both. The system is motivated by the behavior of the alkali metals at significant compression. It accounts for band narrowing, leads to electronic densities with interstitially centered maxima, and can be taken as a model which may be improved upon by perturbation and other methods.