In conclusion, explaining the difference between CO-and HCOOH-producing metal catalysts for CO 2 RR, along with why Ag and Au primarily produce CO, is one of the big gaps in our understanding of the reaction. With this approach, we now have qualitative agreement between simulations and experimental work regarding CO 2 R selectivity. However, there are still open questions regarding the role and species of the bidentate adsorbate on CO-producing metals, and whether it is a spectator or something else. As such, this may not be the complete solution to the question of CO vs. HCOOH selectivity for CO 2 RR, but we believe that it is a valid, empirically based possible explanation that answers many of the questions. Further work would likely include considering how best to falsify this theory, and testing it thus. We should also note that even including water-assisted shuttling and the many different features used for the PCA, our analysis is in no way all-encompassing. As an example, our analysis does not include the effect of cations, despite previous work having shown their their importance for CO 2 RR. 6 Computational Approach