A longstanding challenge in computational heterogeneous catalysis is the inability of density functionals to accurately predict the adsorption energies and preferred adsorption sites of CO molecule on transition metal surfaces, a well-known "CO adsorption puzzle" 44 . For coverages at or below θ < 1/3 monolayer (ML), the top site is strongly favored, as stated in the study by Eve and McCash . At 1/3 ML coverage, CO predominantly occupies top sites, forming the ordered ( √ 3 × √ 3)-30 Previous studies using FNDMC and Random Phase Approximation (RPA) calculations have shown quantitative discrepancies in the adsorption strength of CO at the top site on the Cu(111) surface. Comparisons with experimental zero-point energy (ZPE)-free adsorption energy show that FNDMC closely matches experimental values, while RPA calculations underestimate adsorption energies at the top site (see Figure ).