Our FNDMC models consist of a three-layer copper slab with √ 3 × √ 3 periodicity of the (111) surface at 1/3 ML coverage. We used PBE optimized structures and PBE orbitals as trial wave functions in all FNDMC calculations and computed adsorption energies using Equation 1 for the top, FCC threefold hollow, HCP threefold hollow, and bridge sites: