EA site , PE site , and r d-SA highlight the importance of the electronic properties of the SA and host elements for achieving CO 2 activation. In particular, the rules establish that the SA should display r d-SA > 0.647 Å. This value is larger than the hosts' d-orbital radii equal to 0.319 Å, 0.300 Å, and 0.581 Å, for Cu, Zn, and Pd, respectively. The trend for the d-orbital radius of the elements within a given period decreases as the group number increases. Thus, there is a connection between the uncovered rules and the patterns observed in Fig. , where the SAs with larger d-orbital radius are also the ones favoring larger ∆d C-O max (Os > Ir ≈ Pt, and Ru > Rh). The gen-CN, in turn, denotes the importance of the geometric environment of the surface site as another key factor for CO 2 activation. The rule on gen-CN constrains the values of this parameter to a minimum threshold. This excludes adsorption sites that present unsaturated atoms, such as the ontop and the bridge2-step (see Fig. ) sites of the (211) surfaces, and ontop and short bridge sites of the (110) surfaces. No explicit properties of the host are listed in the rules. However, EA site and PE site introduce a host dependence as these quantities are averages among all the atoms that are part of the adsorption sites (SA + host).