We performed density functional theory (DFT) calculations at the MN15 level of theory in conjunction with SMD aqueous continuum solvation model on computational Ru-CTF model systems to investigate the mechanism of photocatalytic water oxidation reaction (see SI for further details on computational methods). The proposed water oxidation mechanism proceeds via three consecutive oxidation steps to generate reactive Ru V =O species, followed by O-O bond formation and another oxidation step to form the Ru IV -OO species that leads to the evolution of O2, similar to that of the discrete molecular analogue [Ru(tda)(py)2]. Our calculations demonstrate similar redox potentials and reaction energetics for the O2 evolution mechanism of both Ru-CTF model and [Ru(tda)(py)2] indicating similar operating mechanisms (Fig. ).