We present the first unconstrained nonadiabatic molecular dynamics (NAMD) simulations of photocatalytic
water oxidation by small hydrated TiO2 nanoparticles using Tully surface hopping and time-dependent
density functional theory. The results indicate that ultrafast electron–proton transfer from physisorbed
water to the photohole initiates the photo-oxidation on the S1 potential energy surface. The new
mechanism readily explains the observation of mobile hydroxyl radicals in recent experiments. Two key
driving forces for the photo-oxidation reaction are identified: localization of the electron–hole pair and
stabilization of the photohole by hydrogen bonding interaction. Our findings illustrate the scope of
recent advances in NAMD methods and emphasize the importance of explicit simulation of electronic
excitations.