Total energies in Se. III. Defects in the glass

The effective Hubbard $U$ for the bonding coordination defect in glassy Se is investigated. This is accomplished by applying local-density total-energy calculations directly to charged defects in a superlattice configuration. The existence of a large negative contribution to $U$, arising from interconversion between dangling-bond and threefold-coordinated structures, is confirmed. However, a still larger Coulomb repulsion gives rise to an overall positive $U$.