Abstract
The electronic states in sulphur and selenium are investigated by using the nonempirical tight-binding method based on the chemical pseudopotential technique. The equilibrium crystal structures of sulphur and selenium determined by the minimisation of total energies agree fairly well with experimental ones. The neutral onefold-coordinated defect is found to be more stable than the threefold-coordinated one even if the effects of lattice relaxations are considered. Additionally, a quantitative theory of the covalent bonds in these materials based on the localised orbital approach is presented.