CoN is the cobalt nitride stoichiometry with the most favourable formation energy, closely followed by Co 2 N. Given that CoN is the most favourable stoichiometry, the distortion of the Co 100 surface and its thermal instability likely do not arise from the particular stoichiometry. However, bulk CoN has a cubic unit cell and bulk show the same instabilities as Co 100 . Additional analysis on the stable metal nitrides of Ta, Ru, Co and W and the effect this has on the 100% doped surfaces is detailed in Section S6 of the Supporting Information. Table in the Supporting Information shows formation energies, types of unit cell and lattice parameters for selected Ta, Co, Ru and W nitrides.