In this talk I will discuss applications of first principles electronic structure calculations and molecular dynamics simulations to understand materials properties and reaction mechanisms relevant to photocalysis and energy applications. Examples will focus on the surface properties of TiO2, a widely used photocatalyst capable of splitting water in O2 + H2, and  hydrogen  production from water by the active site of an enzyme of hydrogen-producing bacteria, the di-iron hydrogenase, linked to a pyrite electrode.