Bonding Coordination Defect in $g$-Se: A "Positive-$U$" System

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. The existence of a sizable negative contribution to $U$ from structural relaxation is confirmed. However, a still larger Coulomb repulsion gives rise to an overall positive $U$. This result is not inconsistent with a negative $U$ in the compound chalcogenide glasses.