Off-diagonal occupation numbers in local-density theory

We introduce a new method for specifying the occupations of states in local-density theory by allowing "off-diagonal occupation numbers" (i.e., a generalized density matrix). We show that this technique has important applications to superlattice and slab geometries, allowing bonding-type interactions between defects, molecules, or surfaces to be effectively eliminated. Moreover, previously inaccessible charge states may now be studied. The method is shown to have the full sanction of local-density theory.