Full-potential band-structure calculation of iron pyrite

Transition metal disulfides of pyrite structure have recently attracted much interest again due to their large variety of electronic, magnetic, and optical properties. The semiconductor iron pyrite $\left({\mathrm{FeS}}_2\right)$ shows, for instance, an unusual blueshift of the optical gap under pressure. We present a full-potential total energy calculation of iron pyrite using density functional theory with a nonorthogonal local orbital minimum basis scheme. A sophisticated decomposition of the crystal potential and density into a lattice sum of local overlapping nonspherical contributions gives our approach a high numerical efficiency and makes it well suited for open structures like pyrite. For the decomposition of the exchange and correlation potential we introduced a technique of partitioning of unity based on Voronoi polyhedra. We obtain a sufficiently good agreement between our calculations and experimental values for the lattice constant, the positions of the sulfur atoms in the lattice, the bulk modulus, and the frequency of the ${\mathrm{A}}_g$ phonon mode to make a reliable study of the effect of isotropic external pressure on the electronic structure of pyrite and to obtain insight into the optical properties of pyrite.