Surface Structures Similar to the Cu . As for the Cu 2 O surfaces, there seems to be a correlation between the stoichiometric ratio R Cu/O and the magnetic moments of the surface atoms, which are represented by different colors in Figure . If there is an excess of Cu atoms on the surface (R Cu/O > 1), as in CuO(100):Cu, CuO(110):Cu, and CuO(111):Cu, all or some of the outermost Cu atoms do not have a magnetic moment, although all the bulk-like Cu atoms have a magnetic moment, as in CuO itself. On the other hand, if there is an excess of O atoms on the surface (R Cu/O < 1) the outermost Cu atoms have a magnetic moment, which is lower than that in the bulk, e.g., 0. In addition to the most stable antiferromagnetic configuration described above, various other magnetic structures have been studied as well. Here, one diamagnetic, one ferromagnetic, and several antiferromagnetic configurations were obtained for each CuO surface. The corresponding surface energies of the most stable FM, AFM, and DM configuration, respectively, are summarized in Figure , while their surface structures for the overall most stable magnetic configuration are presented in Figure . As shown in Figure , for all investigated CuO surfaces, the antiferromagnetic structure has the lowest surface energy and is, therefore, considered to be the most stable. The diamagnetic structures are in all cases the least stable ones. Similar to Cu 2 O, the energy difference between AFM and DM is most pronounced for the overall most stable CuO(111):Cu/O surface. In contrast to Cu 2 O, the energy difference between AFM and FM is also largest for the most stable surface. The calculated structural and magnetic properties of the CuO(111):Cu/O surface agree well with other theoretical studies. However, to the best of our knowledge, no structural and magnetic properties of the polar nonstoichiometric CuO surfaces investigated in this study were reported previously.