Electronic structure of arsenic chalcogenides

A nonempirical localized-orbital approach is used to calculate the electronic structure of ${\mathrm{As}}_2$ ${\mathrm{S}}_3$, ${\mathrm{As}}_4$ ${\mathrm{S}}_4$, ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$, ${\mathrm{As}}_4$ ${\mathrm{Se}}_4$, and ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ crystals. Contributions of $s$, $p\sigma$, and $p$ lone-pair orbitals to the various molecular levels are illustrated. Densities of occupied states agree closely with experimental photoemission data. The fundamental absorption edges in ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ are found to correspond to indirect gaps, but with several other indirect and direct transitions within a few tenths of an eV of the indirect edge, consistent with most of the optical-absorption studies. The method is readily applicable to the electronic structure of fully coordinated random networks but because of the severe self-consistency problems we are unable to tackle problems associated with thermal relaxation at wrongly coordinated atoms.