This agrees with the experimental result that only the near surface layers are oxidic . Unlike in the reduction process, where the oxygen diffusion from inside to surface was not observed under 300 K when dense copper formed on the surface, here, in the oxidation simulation the oxygen rapidly diffuses from the surface to 6.9 Å inside the surface. The reason may be that copper has enough space to move on the surface. Again, when the oxygen diffuses to 6.9 Å under the surface, the dense copper prevents the diffusion of oxygen, in conjunction with difficulty of depositing more oxygen to the surface, and deep oxidation takes longer.