During structure optimizations, the two bottom layers and the supercell dimensions were kept frozen. We examined four different adsorption sites: atop an O 2-ion, Mg(O), atop a Mg 2+ ion, (Mg)O, a hollow site, and a bridging site between Mg 2+ and O 2-ions. Each adsorption model contains one Co atom per supercell. Figure shows the most stable adsorption system (see Table ), where Co occupies one out of eight surface O 2-ions, namely Co/Mg(O)(001). To model the Co atom in the gas phase, we used a cubic cell of 25 Å edge length.