Atomistic simulation techniques are used to investigate the surface structure, stability and reactivity of pyrite. We introduce a potential model for FeS2 which reproduces experimental structural parameters, elastic constants and hydration energies of pyrite. We modeled the {100}, {110}, and {111} surfaces of pyrite and calculated the {100} surface to be the most stable and to show little surface relaxation, in agreement with experiment. The surfaces were hydrated by associative adsorption of water molecules which stabilized all three surfaces, especially the unstable {111} surface. The calculated adsorption energy of −47 kJ mol-1 for water on the {100} surface agrees well with an adsorption energy of −42 kJ mol-1, determined for the stoichiometric (100) surface by temperature-programmed desorption.1 Adsorption of water molecules at surface sites of lower coordination (four- or three- coordinated) showed increased reactivity of these sites. We calculated an increase in adsorption energy of 50−60 kJ mol-1 ...