Preferred adsorption occurs at the fcc site (-0.58 eV), which is energetically slightly favored compared to the hcp site (-0.56 eV). Thus, both adsorption energies are approximately 40% lower than the corresponding adsorption energies on Mg(101 ! 0)A and Mg(101 ! 1). Clarification may be found again by considering the morphology. Mg(0001) has a flat surface with a high packaging density. The coordination of the adsorbate occurs only to three surface atoms.