3. Why do the Cu-excess Cu 2 O surfaces not exhibit surface magnetism? According to the Bader charge analysis more electrons are localized on the outermost Cu atoms of the Cu-excess Cu 2 O surfaces than on the respective atoms in the bulk phase. We assume that these additional electrons are localized in the 4s orbital, which is in line with our DOS analyses shown in the supporting information (Figure ). Due to the spherical symmetry and nuclear proximity of the 4s orbital, these electrons experience a uniform electric field and a high coulomb force. Moreover, the electrons in the 4s orbital are shielded from the outside by the fully occupied 3d orbitals. Accordingly, we hypothesize that these unpaired 4s electrons do not have a strong influence on the magnetic moment of the respective Cu atoms and thus the Cu-excess surfaces do not exhibit surface magnetism.