Another problem of using this descriptor is the assignment of the correct adsorption site for H. In addition to the general problem of assigning an active site, some arbitrariness arises for example in the question if this should be the most stable site on a chosen facet or the CO 2 RR active site. These findings indicate, that the PZC -which depends on the facet, but is roughly independent of local variations in the field, and thus particular active site environments -might be a much more useful addition to the CO adsorption energy descriptor to rationalize experimental selectivity trends.