The Co 100 surface rearranges to the point where it becomes difficult to describe an adsorption site after relaxation using our labelling. This is similar to Ru 100 in ref . This is most likely caused by the difference in the ionic radius of Co and Ru compared to Ta. Figure compares the Ru, Co and W 100% doped surfaces before and after Cu adsorption. Here, the effect of the difference in ionic radius becomes clear. From largest to smallest the ionic radius of the relevant atoms are: Ta ≳ W > Ru > Co. No rearrangements are observed for W 100 due to the similar ionic radius of W and Ta. A clear rearrangement is observed for Ru 100 , while Co 100 shows the most significant rearrangement due to the large difference in ionic radius. Additionally, without the presence of Ta atoms this "CoN-like" layer begins to distort significantly as illustrated in Figure . Here, we observe extremely large gaps that expose the underlying TaN slab. Additionally, several Co and N atoms migrate away from TaN and adsorb atop the remaining Co and N atoms to form a second layer of CoN (see Figure ). These rearrangements are most likely caused by the lattice mismatch and different unit cells for CoN (cubic) and TaN (hexagonal). More details on this are included in Section S6 of the Supporting Information.