The electronic properties of the CuO surfaces were deduced from calculated spin-polarized DOS. The total, partial, and projected DOS of the most stable CuO(111):Cu/O surface are shown in Figure , while the DOS for the other surfaces is given in the supporting information (see Figures and). For the stoichiometric CuO(111):Cu/O surface, there is no difference between the DOS for spin-up electrons (plotted upwards) and the DOS for spin-down electrons (plotted downwards). Compared to the CuO bulk phase, additional states are observed near the conduction band minimum (around 1.3 eV), which are magnified in the inset. These states can be attributed to the surface atoms and exhibit both Cu-3d and O-2p character, with the density of the Cu-3d states being greater than the density of the O-2p states. Similar to the Cu 2 O(100):O surface, this 2p-3d hybridization can be observed not only for the surface atoms but also in the bulk. The projected DOS shows that the states from -4.5 eV to the valence band maximum have both O-2p and Cu-3d contributions. However, the density of the Cu states decreases in this range, while the density of the O states remains almost constant. Consequently, the states above -2 eV can be assigned predominantly to the O-2p orbitals. On the other hand, the states below O surfaces have either an excess of spin-down or spin-up electrons at the surface, which can also be deduced from Figure . Therefore, the respective DOS show different characteristics for spin-up and spindown electrons. To gain further insight into the electronic structure of the surface atoms, we additionally performed a Bader charge analysis for all investi-gated CuO surfaces, as shown in the supporting information (Figure ). For all surfaces, the charge of the atoms inside the slabs matches the charge of the respective atoms in the bulk phase, namely 1.1 e for Cu and -1.1 e for O atoms, respectively. In the bulk phase, as well as inside the slabs, each Cu atom is coordinated with four O atoms and each O atom is coordinated with four Cu atoms. This indicates that on average 0.275 e is transferred from each Cu atom to each neighboring O atom. For the Cu-terminated surfaces, the calculated Bader charges of the outermost Cu atoms are about 0.5 e. Accordingly, the charge of the outermost Cu atoms of these CuO surfaces is similar to the charge of the Cu atoms in the Cu 2 O bulk phase. This indicates that the outermost Cu atoms of the Cu-terminated CuO surfaces are reduced (from Cu 2+ to Cu + ). This interpretation is in line with Maimaiti et al., who also suggested that a change in charge of 0.5 -0.6 e indicates a reduction from Cu 2+ to Cu + . For the O-terminated surfaces, only the charges of the outermost O atoms differ from the respective bulk values. Although there is a deficiency of electrons on the O-excess CuO surfaces compared to the CuO bulk phase, the charge of the Cu atoms does not change compared to those in the bulk phase. This indicates that the Cu atoms transfer a maximum of 1.1 e regardless of their coordination environment, and thus, none of the Cu atoms have an oxidation state larger than that in CuO.