  We have used a density-functional-based tight-binding method in order to
create structural models of the canonical chalcogenide glass, amorphous
(a-)As$_2$S$_3$. The models range from one containing defects that are both
chemical (homopolar bonds) and topological (valence-alternation pairs) in
nature to one that is defect-free (stoichiometric). The structural, vibrational
and electronic properties of the simulated models are in good agreement with
experimental data where available. The electronic densities of states obtained
for all models show clean optical band gaps. A certain degree of electron-state
localization at the band edges is observed for all models, which suggests that
photoinduced phenomena in chalcogenide glasses may not necessarily be
attributed to the excitation of defects of only one particular kind.
