A theoretical study of selenium I under high pressure

Using recently developed codes for density-functional total-energy calculations we trace the structural and electronic response of the hexagonal phase of selenium to applied pressure. We find that the anomalous linear expansion coefficient is well reproduced, and the structure reduces its volume by straightening its twofold-coordinated chains and bringing them closer together. The characteristic overbinding of the local-density approximation causes an effect akin to a spurious pressure on the system rather than a straightforward volume rescaling. The model also predicts a band gap closing rapidly with pressure within the same structural space group. This is not the observed metallization pressure, which in practice is induced by a structural phase transition. We further show that the valence bands are correctly associated with covalent bonds, lone pairs and s-type atomic orbitals, with the lone pairs being the least strongly bound.