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.