  We report on density-functional-based tight-binding (DFTB) simulations of a
series of amorphous arsenic sulfide models. In addition to the charged
coordination defects previously proposed to exist in chalcogenide glasses, a
novel defect pair, [As4]--[S3]+, consisting of a four-fold coordinated arsenic
site in a seesaw configuration and a three-fold coordinated sulfur site in a
planar trigonal configuration, was found in several models. The
valence-alternation pairs S3+-S1- are converted into [As4]--[S3]+ pairs under
HOMO-to-LUMO electronic excitation. This structural transformation is
accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests
that such transformations could contribute to photo-darkening in these
materials.
