Due to the smaller ionic radius of Co (0.55 or 0.61 Å, depending on high or low spin) compared to Ta (0.72 Å), extremely strong surface rearrangements were observed on the Co 50-S , Co 50-F and Co 100 surfaces. For this reason, the adsorption energy and the Cu-Cu interaction energy for these surfaces were computed by using E surf , the single point energy of the rearranged surface after removing Cu instead of E bare , the total energy of the relaxed surface before metal adsorption, as a reference. This is the same energy used to compute E Cu-substrate , which has the consequence that the Cu-Cu interaction energies are based solely on the difference in the Cu reference energies.