In the unsolvated (vacuum) case, the difference in FNDMC adsorption energy be- 0.67 eV unsolvated in favor of CHO*) (see Table ). These findings are in good agreement with previously published high-level embedding methods , which report similar stabilization energies of 0.25 eV in favor of COH*. The consistency between our FNDMC results and these high-level methods validates our approach and underscores the importance of including solvation effects when investigating the energetics of intermediates in CORR on Cu(111) surfaces.