Total energies in Se. II. Vacancy in the crystal

Ab initio total-energy calculations are used to determine theoretically the structural configuration of the vacancy in trigonal Se. The method consists of calculating the forces, as well as the total energies, within the local-density and frozen-core approximations, for a superlattice structure containing a vacancy. In this way, relaxations at the vacancy can be fully taken into account, including a possible self-healing of the vacancy. A slightly relaxed symmetric version of the ideal vacancy is the lowest-energy structure found; neither asymmetric relaxation nor valence alternation appears to occur. A simple Hubbard Hamiltonian is used to analyze the spin configuration of the lowest-energy structure.