As a first step, we calculated the partial DOS for all three configurations (atop, bridge, and fcc hollow) at θ = ¼ coverage. Fig. shows the result together with levels extracted from PE measurements 13 overlaid in red. As the PE measurements were taken for θ = ½ samples, they are not directly comparable to the calculated results. The comparison is nevertheless useful because it demonstrates that both occupied states and the LUMO 2π* state are sensitive to configuration, and therefore the optical data can serve as a useful structural probe. For PE data 13 , the authors report equal occupation of atop-and bridge-CO sites. If we accept this to be correct, the closest comparison to experiment would be to average the calculated PDOS in the left and center panels in equal measure. On the other hand, if the total energies of Table are accepted, the relative populations of the atomic and bridging site will differ by a Boltzmann factor, exp[-ΔE/kBT]. Taking into account the large uncertainty in ΔE (Table ) the Boltzmann factor ranges between 0.21 for ΔE=0.02 eV to 0.004 for ΔE=0.07 eV. It is very likely, therefore, that the atomic configuration is much more populous than the bridging one. This is consistent with QSGW calculations which align very well with PE data for the atop site, and much less so for the other configurations. Below we compare this data to calculations with θ = ½ coverage, and this will help to elucidate the experimental findings.