Here we use a self-consistent form of many-body perturbation theory (the quasiparticle self-consistent GW approximation-QSGW 6 ), which has much higher fidelity than DFT, and also higher fidelity than the GW approximation based on DFT. QSGW accurately captures valence d bandwidths, magnetic moments, and the positions of s-and d-bands in 3d metals. It has also been applied to the GW100 set of molecules and shown to yield very good ionization potentials. . In a study of the Fe/MgO magnetic tunnel junction (a system with some similarities to the present work), it gave an excellent description of the transmission probability. For benchmarking adsorption of CO on TM surfaces, we take structures of the adsorbed molecule using DFT, and perform QSGW calculations on them. We compare to photoemission data for CO on Cu (111) and Pt (111) . Pt (111) as they remain the best characterized experimental studies of CO preferentially adsorbing on an atomic site, whereas Cu (111) is an experimentally relevant catalyst for electrochemical CO2 reduction. The theoretical insights to CO in reference to photoemission data will be a significant first step to enabling more accurate predictions of the mechanisms for CO2 reduction and may be especially relevant for electrochemical, photoactive, and plasmonic applications. .