Here E O * , E H2 , E * , E H2O are the DFT energies of the surface slab with the oxygen species, a hydrogen molecule, the surface slab without the oxygen species and a water molecule respectively. The oxygen binding energy can indicate the stability of the bound oxygen, and is thereby a useful indicator of whether it may be expected to be active. Initial analysis of the pure (i.e. non-doped) RuO 2 surface is presented in fig. . Since an oxygen intermediate is the second step of conventional four step single site OER mechanism, the ideal catalyst would have the oxygen intermediate binding energy being 4.92eV /2 = 2.46eV . With this in mind, the bridge site oxygen binding energy being 0.56 eV indicates that it is strongly bound, and would not be expected to evolve oxygen. Experimentally RuO 2 is a highly active catalyst, with the CUS site being the commonly anticipated active site. Therefore, at first glance you would expect active oxygen in/on the lattice to be bound with a similar binding energy as the CUS-bound oxygen (≈1.89 eV). Compared to the oxygen binding energy of the ideal catalyst, 1.89 eV is too strongly bound for it to be an efficient catalyst if it is to follow the conventional single site mechanism (similar to iridium oxide). The resulting activity gap between theory and experiments has previously been explained by considering an alternate reaction pathway on the surface in which the proton in intermediates * OH and * OOH migrates to the bridge position. This reaction mechanism was first proposed for systems including dopants in the bridge , but has since been found to alleviate the aforementioned activity gap for pure RuO 2 . The pure rutile surface serves as a reference for binding energies of the oxygen intermediates when one exchanges metal atoms in the lattice. It would generally be expected that active lattice oxygen following a single-site mechanism would be at least somewhat weaker bound than the 1.89 eV presented by the CUS site. It needs to be noted that only this CUS site oxygen is bound weakly enough to be evolveable on the pure surface. It is therefore to be expected that this surface does not evolve lattice oxygen. This is in line with the more recent experimental findings in the literature .