Next, we consider the θ = ½ coverage by co-adsorbing a pair of CO molecules on the surface, which is better reflective of the experimental configurations. We relaxed the atomic + bridging, and atomic + fcc hollow pairs with the LDA, PBE, and BEEF-vdW functionals, as we did for the θ = ¼ configurations. In the course of the relaxation, a new configuration (the OCCO dimer) emerged spontaneously. Fig. presents cartoons of the resulting structures along with the Bader charges and energetics. The relative energies of these isomers can be sensitive to the magnitude of charge transfer between atoms: for the BEEF-vdW functional, there are two minima for the atomic + bridging configuration with a relative energy of 0.03 eV between them. Geometrically, they are nearly identical, but the slightly more stable isomer features more extreme positive (negative) charges on the C (O) atoms for the CO on an atop site. Therefore, they may be considered two unique minima due to the degree of charge transfer between the C-O atoms. Bond lengths are compared to experimental data in Table . The atomic + fcc hollow configuration remains the most stable across the different functionals although the relative energy of the co-adsorbed atomic and bridging site from the atomic and fcc hollow site decreases from the LDA to the PBE to the BEEF-vdW functional. Also Bader charges in the 2-CO geometries fluctuate less across functionals, with the majority of the CO molecules exhibiting a slight negative charge. The C-O and Pt-C bond lengths of these co-adsorbed geometries reproduced similar values to those found at θ = ¼ coverage and therefore complement experiment (Table ). While LDA and PBE functionals found the OCCO dimer to be a 1 eV higher than other co-adsorbed geometries; this difference was smaller with BEEF-vdW with only 0.36 eV relative to the co-adsorbed atomic + fcc hollow site. This highlights the dependence of the functional on the relative energy of possibly catalytically relevant species. A DFT study exploring CO dissociation routes on Pt and Pt-Sn also found the OCCO dimer, but determined the high formation barrier to be an energetically unfavorable mechanism. However, this conclusion may change, as it was predicated on a barrier that is likely too large. As observed in Figures and, the energies of various minima can differ substantially, and BEEF-vdW has thus far drawn closest to the relative energies in experiment of the different sites (see Table ). Therefore, there is some ambiguity on the energetic possibility of the OCCO dimer. In Figure , we visualize the PDOS of the co-adsorbed atomic + bridging sites, atomic + fcc hollow sites, and the OCCO dimer. Comparing to Figure , the influence of CO-CO on the position of the molecular levels can be seen. All three levels (4σ, 5σ, and 1π) below EF align very well with respect to experiment. This provides the best benchmark of the theory, as CO configurations are the closest to the experimental ones. The 4σ contains strong contributions from the Pt s-and d-orbitals, O s-and p-orbitals, and C s-orbital. For the 2π*, a smearing effect occurs within the 4.2-4.5 eV range found by Anazawa et. al. for the bridging and atomic sites. In contrast, the PDOS for the atomic + fcc hollow site exhibit an intense peak at 4.2 eV with a small shoulder at 4.5 eV above EF. This suggests that interactions between a CO molecule on an atop site and a CO molecule on a hollow site may have contributed to the 4.2 eV peak observed in the 2PPE experiments by Anazawa et. al.