There is one important contrast between Pt and Cu cases: the 2π* LUMO has a moderately sharp peak only for the atomic site (2π* at 3.6 eV, close to experiment for lower coverages). For the other sites, the level broadens into a band approximately 4 eV wide. This is likely because for the atomic site, the C 2px O 2px and Cu 3dxz form a linear chain of orbitals with π character that does not occur for the other geometries, which provides a more favorable environment for directional molecular bonds. At higher coverages, θ = ½, we consider CO co-adsorbed on the atomic + bridging sites, atomic + fcc hollow sites, atomic + atomic sites (Figure ). BEEF-vdW adsorbed CO weakly compared to experiment; the ramifications of this results in the neighboring CO desorbing from the surface. Moreover, the LDA and PBE functionals often relaxed the bridging site lower along the z-axis to resemble the hollow site. Bader charge analysis reveals that CO is slightly electronegative relative to Cu. C-O bond lengths remained similar when relaxed under the different functionals, with Cu-C bonds exhibiting a greater range (1.87-1.97 Å for PBE's atomic site; 1.92-1.97 Å for BEEF-vdW), probably originating from steric interactions between neighboring CO molecules. While the OCCO dimer was easily found on Pt (111), it was difficult to stabilize on Cu (111). This OCCO dimer is stable only for LDA and PBE functionals; in the BEEF-vdW case it would instead relax to co-adsorbed atomic + atomic sites. However, this may be in keeping with recent experimental-theoretical studies on the OCCO dimer on various facets of Cu. In situ Raman spectroscopy combined with DFT calculations have attributed the OCCO dimer to be a key reaction intermediate in the selectivity of C2 products, and it may explain the unique reactivity of Cu (110) over Cu (111). Similarly, Fourier transform infrared spectroscopy detected the presence of the hydrogenated CO dimer on Cu (100) and not on Cu (111). Highlighting that this may also explain Cu (100)'s high reactivity compared to Cu (111). Photoelectron spectroscopy of Ref 10 was measured for the relatively low coverage of θ = 1/3, but by comparing θ = ¼ to θ = ½, we can probe CO-CO interactions. Theory predicts two peaks straddling the observed EF -11.6 peak. Similarly, theory predicts EF -8.5 peak to consist of multiple states straddling the observed level (see Figure ). For both peaks, the average of the theoretical peaks is slightly higher than the experiment, and offset slightly from the theoretical average of θ = ¼ coverages. If the theory is a reliable predictor of these levels, this offset may be interpreted in part as the experimental geometry consisting of an asymmetric occupation of (majority) atomic sites and (minority) bridging sites. Moreover, the offset is a signature of CO-CO interactions. It appears to be a steric effect: the CO molecules are slightly angled away from each other.