Internal relaxation, band gaps and elastic constant calculations of FeS2

Full relaxation of volume and internal parameters of iron pyrite and marcas ite FeS2, has been studied using a plane-wave pseudopotential method within the Local Density Approximation to Density Functional Theory (LDA-DFT). We find that the internal parameter u of pyrite decreases with hydrostatic co mpression. The P-V equation of state falls slightly closer to the experimen tal curve than a previous unrelaxed Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) calculation. The optimized parameters are used in a TB-LMTO calc ulation to predict the electronic structure from which we find a larger ban d gap on marcasite compared to pyrite. Ab initio calculations of elastic co nstants for pyrite were performed using the Full Potential (FP) LMTO method and agree to within 7% with experiment.