The highest energy gain results from adsorption on the bridge-bottom position on Mg(101 ! 0)B (-1.64 eV), which is also the only stable adsorption site on this surface. The high adsorption energy seems logical since Mg(101 ! 0)B, the surface termination with the highest surface energy, aims for its stabilization by a transformation into the much more stable counterpart Mg(101 ! 0)A. In fact, this becomes even more evident when comparing the structural environments of the adsorption sites in SI There are two stable adsorption sites in the case of the most stable surface Mg(0001).