We now turn to CO on Cu (111), a catalytically relevant material for CO2 reduction. Less experimental data is available for the Cu (111) case. This is most often attributed to the weak binding of CO (-0.52 eV), which decreases with increasing coverage of CO to -0.39 eV (see Table ). In an IRAS study comparing atomic, bridging, and hollow sites, Hayden, et al. found only around 20 cm -1 difference in the IR bands between the bridging and hollow sites. They concluded that the energetic potential well between the two states to be shallower than the corresponding 0.04 eV difference on Pt (111) (see Table ). In Tables and, we summarize the DFT-level energies and bond lengths as compared to experiment; in Figure , these results are visualized. Unsurprisingly, the LDA functional resulted in the strongest binding energy, followed by the PBE functional. Regardless of the functional, Bader charges found CO to be slightly negative, ranging from -0.05 to -0.48 e. While C-O bond lengths remained similar across the different functionals, the Cu-C bond length variation was significantly wider than the Pt-C bond lengths in the previous section. The BEEF-vdW functional slightly underestimates adsorption energy, but predicts bond-lengths closest to experiment and correctly predicts the most stable adsorption site to be CO atop a Cu. Therefore, we used BEEF-vdW geometries in subsequent self-consistent GW calculations. Exp, Eads (eV) -0.52 (θ ≤ 1/3), -0.39 (θ > 1/3) 38