Pairing of Electrons by a Point Defect in $c-{\mathrm{As}}_2{\mathrm{Se}}_3$

By use of ab initio density-functional total-energy calculations a point defect with a negative Hubbard $U$ is identified, for the first time, in a chalcogenide system. It is found that in $c-{\mathrm{As}}_2{\mathrm{Se}}_3$ two noninteracting neutral Se antisite defects are unstable toward the formation of a pair of noninteracting oppositely charged Se antisites. The correlation energy gained is 0.3 eV per electron pair.