Our calculations reveal only negative or negligible (|Eint| < 0.03 eV) interaction energies between two Mg adsorbates and an interaction sequence given by Eint, Mg(0001) > Eint,Mg(101̅ 1) > Eint,Mg(101̅ 0)A, as shown in detail in Table , Table and Table . The corresponding dimer sites are given in Figure . The differences in interaction energies may be related directly to the number of nearest surface atoms of the adsorbates following the opposite order (3Mg(0001) < 4Mg(101̅ 1) < 5Mg(101̅ 0)A).