Interatomic potential parameters have been derived at simulated temperatures of O K and 300 K to model pyrite FeS2. The predicted pyrite structures are within 1% of those determined experimentally, while the calculated bulk modulus is within 7%. The model is also able to simulate the properties of marcasite, even though no data for this phase were included in the fitting procedure. There is almost no difference in results obtained for pyrite using the two potential sets; however, when used to model FeS2 marcasite, the potential fitted at O K performs better. The potentials have also been used to study the high-pressure behaviour of pyrite up to 44 GPa. The calculated equation of state gives good agreement with experiment and shows that the Fe-S bonds shorten more rapidly that the S-S dimer bonds. The behaviour of marcasite at high pressure is found to be similar to that of pyrite.