On Mg(0001), only two possible dimer conformations could be detected, which are both shown in Figure . If both atoms were initially placed in hcp or fcc positions, the relaxed dimer ends up with both atoms in the same bridge-like position 0 ↔ 1 with an interaction energy of 0.51 eV. In the thermodynamically more stable dimer conformation 0 ↔ 6 (0.54 eV), one atom is located in an fcc or hcp position above a surface atom, respectively. The dimer configurations 0 ↔ 4 and 0 ↔ 7 are not stable and end up in dimer configuration 0 ↔ 6. Two conformations with noticeable interaction energies are also present on Mg(101 ! 1). In the thermodynamically more stable conformation, both adsorbates are in adjacent hollow positions (0 ↔ 3) and have an interaction energy of -0.31 eV. In the thermodynamic less stable conformation, one atom sits in a hollow-and the other in an fcc-like position (0 ↔ 1). Since the atom sitting in fcc position coordinates to only three surface atoms, the interaction to the second adsorbate is slightly higher (-0.35 eV). For the BEEF-vdW functional, the differences in interaction energies are less pronounced for the two dimer conformations on Mg(101 ! 1). At greater atom distances, the interaction energy drops for all investigated surfaces to negligible values. Nevertheless, the PBE functional values are consistent with the BEEF-vdW calculations and literature. fcc 0 -0.50 , 0 ⟷ 2 -0.05 -0.02 (0.12) 0 ⟷ 3 -0.04 0.00 (0.09) 0 ⟷ 4* -0.54 -0.43 (0.11) -0.05 0 ⟷ 5 -0.03 0.01 (0.13) 0 ⟷ 6 -0.54 -0.43 (0.11) -0.50 0 As the interaction energy studies (Figure , Table ) have already shown, the diffusion barriers reveal that when two adsorbates meet on Mg(0001), they immediately agglomerate [D0/2 → D1/3, Ea for = 0.00 eV]. We could not detect a barrier below a certain distance. On the contrary, the activation energies for its separation are multiple times higher [D1/3 → D0/2, Ea rev = 0.49/0.46 eV], indicating that one dimer can already be considered a center for further island growth. Also noteworthy is that from the time of dimer formation, the propagation as a dimer on Mg(0001) is higher [CD0 ↔ CD1, Ea = 0.07 eV] than that of the monomer still, the dimer is highly mobile. The transition between the observed dimer configurations occurs almost barrierless [CD0 ↔ CD2, Ea = 0.01 eV].