Cuprous oxide (Cu 2 O) crystallizes in a primitive cubic Bravais lattice (space group P n 3m). The unit cell, shown in Figure (left), consists of two formula units, i.e., four Cu and two O atoms. The Cu atoms (large dark grey spheres) are linearly coordinated to two O atoms (small light grey spheres) and arranged in a facecentered cubic (fcc) sublattice, while each O atom is tetrahedrally coordinated with four Cu atoms, forming a body-centered cubic (bcc) sublattice. Both types of atoms in Cu 2 O have no local magnetic moment. Thus, the material is diamagnetic (DM). As shown in Table , the calculated lattice parameters, energy of formation ∆ f G°, and magnetic structure (DM) are in excellent agreement with the experimental results. However, the band gap E G of Cu 2 O is underestimated, which is well known for standard DFT and has already been discussed elsewhere. . Additional antiferromagnetic configurations with different arrangements of the magnetic moments were investigated and are shown in the supporting information (Figure ). Although the energies of formation of these AFM configurations are very similar, the magnetic structure observed experimentally (Figure ) is also the most stable in our study. For this surface, we observed a surface reconstruction, whereas Yu et al. reported only small changes in bond lengths on the surface. A possible explanation for this discrepancy could be that we used more rigorous convergence criteria for the geometry optimizations, e.g., 0.001 eV Å-1 instead of 0.02 eV Å-1 . Surface Magnetism Except for the stoichiometric Cu 2 O(111):Cu/O surface, all Cu 2 O surfaces have either an excess of Cu or O atoms on the surface and are therefore considered as polar surfaces. The excess of Cu or O atoms can be described with the stoichiometric ratio Depending on the arrangement of these magnetic moments, different magnetic structures can occur. Figure shows the surface energies of the Cu 2 O surfaces with an excess of O atoms for the ferromagnetic (FM), diamagnetic (DM), and most stable antiferromagnetic (AFM) configuration, respectively. The corresponding surface structures within their most stable magnetic configuration are shown in Figure . According to Figure , for all surfaces with an excess of O atoms, the FM configuration has the lowest surface energy and is, therefore, the most stable one. The AFM structure is the second most stable for all of these surfaces, while the DM structure is the least stable. For the Cu 2 O(111):O surface, the surface energy difference between the FM and AFM structures is subtle, indicating that both structures are , while the DOS for the other surfaces is given in the supporting information (Figure and Figure ). The spin-up states of the valence band are fully occupied, while the spin-down states intersect the Fermi level, indicating partial occupancy and half-metallic behaviour. These spin-down states near the Fermi level can be assigned to the surface atoms (outermost atomic layers) and have both Cu-3d and O-2p character with similar density contributions, as shown by the partial and projected DOS. This 2p-3d hybridization can be observed not only for the surface atoms but also for the bulk-like atoms.