Using clusters of atoms that model the local structure of amorphous As2S3 (a-As2S3), we here present a model of photoinduced structural changes in a-As2S3. We have performed quantum-chemical calculations on the model clusters and have obtained their equilibrium configurations, charge distributions, molecular-orbital structures, and excitation energies. It has been found that there exist at least two types of metastable structural defects. One has a fivefold-coordinated As unit, and the other comprises a fourfold-coordinated As atom and a nonbridging S atom. Each type of defect results from the breaking of an As-S bond and subsequent structural rearrangement. From the calculations of the excitation energies, we suggest that the fivefold As defect is responsible for a parallel redshift of the optical absorption edge upon light exposure, called photodarkening. On the other hand, the fourfold As defect is likely to contribute to the photoinduced midgap absorption below ~2 eV induced by low-temperature light exposure. It has also been demonstrated that the formation of these metastable defects is closely associated with interlayer atomic reconfigurations resulting in homopolar As-As linkages.