On the other hand, this contrasts with the hcp packaging series of metallic Mg that should follow the ABABAB-stacking sequence. Two questions arise, first, why are fcc sites favored over hcp sites for smaller islands, and second, at what critical island size does the original packing order become thermodynamically more stable again. Therefore, for islands up to a size of 7 atoms and full monolayers (36 atoms) on (6×6) Mg(0001), the adsorption energies per atom were calculated as shown in SI Figure . Additionally, the exact adsorption energies per atom for the islands and for full hcp and fcc monolayers, as well as the stacking fault energies, may be found in SI Table VII. It turns out that starting from a heptamer adsorption is slightly favored on hcp sites, while tetramers, pentamers, and hexamers are indifferent. This finding is consistent with the results of Ortigoza et al. The group confirmed stacking fault up to the trimer but also used only half of the unit cell size ((3×3) vs. (6×6)). They blame Friedel oscillations for inducing an increased charge density at the fcc pocket, which noticeably stabilizes the bond of the adsorbate to its nearest neighbors. At the same time, however, the increased charge density at adjacent fcc positions vanishes, which explains the renewed preference of the hcp position for larger adislands. On the second studied surface Mg(101 ! 1), the barriers for trimer formation are generally higher than on Mg(0001). In addition, the linear trimer form is thermodynamically more stable with respect to the triangular trimer form. If an atom encounters a dimer within a channel, the Step, kink and corner self-diffusion on Mg(0001) and Mg(101 ! 1)