The (111) surface was cut from a bulk unit cell of experimental lattice constant 3.9231 Å for Pt and 3.61496 Å for Cu. For CO adsorption on Pt (111) and Cu (111), we utilized a 4-layer surface cell with 4 surface metal atoms exposed; for co-adsorption of 2 CO molecules on Pt (111) and Cu (111), we utilized a 6-layer surface cell with 4 surface metal atoms exposed. A vacuum gap of double or more the height of the surface was incorporated in order to prevent spurious interactions between the periodic images. Subsequently, the calculated adsorption energies utilized the following equation: Eads = Esurf+ads -Esurf -Egas,CO, where Esurf+ads was the total energy of the surface with adsorbate; Esurf was the total energy of the clean surface without adsorbate; and Egas,CO was the total energy of the gas phase CO. Post-processing of isomers was performed, extracting electronic information for bonding analysis such as charge-transfer via the Bader charge algorithm and orbital overlap from projected density of states. For geometric relaxation, the plane-wave DFT code, the Vienna Ab-Initio Simulation Package (VASP 5.4.4) was utilized. The planewaves basis set was expanded to a kinetic energy cut-off of 520 eV, incorporating optimization of geometries to a convergence criteria of 10 -6 (10 -5 ) eV for electronic (geometric) relaxation and Gaussian smearing to account for the complexity of metal-adsorbate properties. A dense k-point mesh of 12 × 12 × 1 was implemented. For subsequent quasiparticle, self-consistent GW-level calculations, BEEF-vdW geometries were computed with an 8 × 8 × 1 k-point mesh for the one-body part and a 4 × 4 × 1 k-point mesh for the two-body part. The cutoffs to the GW basis sets were set at 5.0 a.u. for CO on Cu (111) calculations and 4.5 a.u. for CO on Pt (111) calculations. The self-consistency cycle was iterated until the change in self-energy fell below 10 -5 . QSGW calculations were performed with the Questaal package.