After annealing, most of the oxygen would be located close to the surface to form the structure of Cu2O(111) if the remaining number of oxygen atoms fits the capacity of the surface. Still, the reason could be that Cu2O(111) has lower surface energy than Cu(111), and the formation of Cu2O on the surface can reduce the energy of the system. But few oxygen atoms could be trapped inside, Figure . If the amount of oxygen atoms exceeds the capacity of the surface, the excess oxygen atoms aggregate in the bulk to form Cu2O grains, consistent with the experimental results. The oxygen depth profile shown in Figure for the low specific surface area model (S.S.A. = 0.034 Å -1 , Figure ), shows three peaks corresponding to Cu2O, two on the surface and one inside the bulk. The peak inside the bulk indicates the formation of a Cu2O layer (4 atoms per 1.08 Å for Cu2O(111)-p(2×2)) parallel to the surface as Cu2O has a lower formation energy than CuxO ( x =1, 6, 8, 64) . Therefore, oxygen atoms tend to aggregate to form Cu2O to reduce the overall formation energy, and near the surface to further reduce the surface energy. Thus, considering a large size OD-Cu catalyst containing enough O, the stable static configuration should be that part of oxygen atoms are located on the surface to form Cu2O surface. Meanwhile, excess oxygen leads to Cu2O formed inside the bulk segregated from the Cu crystal. The diffusion and aggregation of oxygen were also observed in the NVT simulation at 300 K showed upon long-time equilibration (100 ns), Figure . The two peak inside the bulk at 10 ns correspond to the two types of oxygen proposed by a recent XPS study , Figure .