The presence of adsorbed oxygen significantly alters the energy landscape for water dissociation. Energy barriers for water dissociation are +0.10 eV for oxygen-assisted dissociation at bridge sites, +0.59 eV on the bare Fe(100) surface, and +0.96 eV for oxygenassisted dissociation at hollow sites. The bare Fe(100) surface requires a moderate amount of energy (+0.59 eV) for water activation, suggesting that the iron surface can readily facilitate water dissociation. However, the water dissociation energy obtained here was much lower than the values reported in previous metal/gas phase studies of ~ 1 eV. This is likely due to the extra stabilization provided by surrounding water molecules to the transition state, which was not considered in previous studies. The lower energy barrier at the bridge sites (+0.10 eV)