As noted in numerous IRAS studies, the hollow site appears at circa θ = 0.3 and differs in energy from the bridging site by only 0.04 eV. Much of the focus of previous theoretical studies was on first achieving the correct adsorption energy and site geometry and secondarily, achieving the HOMO-LUMO gap. By also considering the θ-dependence of the 4σ, 5σ, and 1π levels and the LUMO 2π*, we can directly connect to experiment. Moreover, the benchmark just described establishes that QSGW is able to accurately predict these levels. Assuming a similar accuracy for other interfaces, this approach's predictive power provides a powerful complement to the optical experiments. It provides a one-to-one correspondence between spectra and structure, and can aid in the search for particular structures via photoemission experiments. The OCCO dimer provides an excellent illustration of this. For Cu substrates, it is thought to be an important intermediate state in the catalysis process . According to QSGW, its signature is markedly different from the atomic+bridging sites or atomic+fcc hollow sites, especially the shift in the 4σ state and the splitting of the 2π* peak to two states near 3 and 5 eV.