Density-functional calculation of the electronic structure and equilibrium geometry of iron pyrite (FeS2)

The electronic structure and equilibrium geometry of the pyrite form of ${\mathrm{FeS}}_2$ was studied using self-consistent density-functional theory comparing the local-density-approximation (LDA) and the generalized-gradient-approximation (GGA) forms of the exchange-correlation functional. The calculated contour map of the electron-deformation density is consistent with analysis of high-resolution x-ray-diffraction data, showing excess charge in nonbonding d states on the Fe sites. At the experimentally measured geometry, the cohesive energy is calculated to be 16.7 eV/${\mathrm{FeS}}_2$ and 9.4 eV/${\mathrm{FeS}}_2$ for the LDA and GGA forms, respectively; the GGA result comparing well with the experimental value of 10.7 eV/${\mathrm{FeS}}_2$. Optimizing the cohesive energy as a function of geometry, both the LDA and GGA calculations find the optimal structure to be expanded relative to the experimental geometry with the lattice constant expanded by 1% and the S-S bond length expanded by 6%, corresponding to a 0.1-eV increase in the cohesive energy for each ${\mathrm{FeS}}_2$ unit. Similar results were obtained for ${\mathrm{RuS}}_2$.