Table compares the total energy for the three absorption sites, along with measured values taken from He scattering for desorption of CO and IRAS-LEED. Notably, the functionals differ considerably in both their reaction energies and their relative change in energies across the three adsorption sites. BEEF-vdW performed best for relative energies, with ΔE ranging between 0.00-0.11 eV, compared to measured values of ΔE = 0.00-0.11 eV. This is a marked improvement over LDA (ΔE = 0.00-0.33 eV) and PBE (ΔE = 0.00-0.19 eV) Even for BEEF-vdW, some discrepancies with experiment are seen. It finds the bridging site to be more stable than the atomic site by 0.11 eV, while experimentally this difference is -0.07 eV. Subsequent QSGW calculations were evaluated for structures derived from the BEEF-vdW functional, since it performs best. From QSGW we can evaluate projected density of states (PDOS) to compare to experimental photoemission data. 13, 14 PDOS allows us to elucidate the s-, p-, d-orbital contributions by Pt, O, and C and correlate their contributions to the σ-and π-levels determined in experiment. Experimental values of CO-derived σ-and π-levels below the Fermi energy EF on Pt (111) are reported for θ = ½ coverage, with an equal amount of atop-and bridge-CO sites reported to be occupied. More indepth IRAS studies observe that due to the 0.04 eV energy difference between the bridging and hollow sites, Boltzmann populations factor in the occupation of the hollow sites. Even at temperatures as low as 20 K the hollow site's IR-band appears at θ = 0.35. For occupied σ and π CO states, Tsilimis et. al performed an ultraviolet photoemission spectroscopy (UPS) study at 150 K, while Anazawa, et. al. determined the position of the unoccupied LUMO 2π* states from a two-photon photoemission (2PPE) study at <120K. While atomic and bridging sites should be dominant, some non-negligible percentage of CO that occupies hollow sites is also likely. eV, 5σ at -9.3±0.2 eV and 1π at -8.4±0.4) 13 , while above EF data was taken from a two-photon experiment from Anazawa, et. al (LUMO state for atomic site at 4.5 eV at <0.17 ML) 14 Pink shaded areas reflect uncertainties reported in the measurements. All three figures use the same experimental data. The abscissa was shifted to place the Fermi energy at 0.