and Mg(110), since it is difficult to calculate the surface energy for the non-stoichiometric surface MgO(111). Either the stoichiometric structure has different (oxygen-rich on the one side, magnesium-rich on the other side) terminations, or it is impossible to reference the symmetric structure against the bulk energy. Reuter and Scheffler stated in this context that "in a purely electrostatic model in which all oxygen ions would be in their bulk formal oxidation state, polar surfaces would be charged and should thus exhibit an infinite surface energy". The surface energy results for MgO(100) and MgO(110) were calculated using Equation ( ) and are shown in Table . According to the DFT data set, the tetrahedral position tet-1 between the first and second surface layer is the most stable adsorption site on Mg(0001), which is in agreement with the results of Francis and Taylor. However, our adsorption energy of -4.42 eV is significantly less exothermic (vs. -7.69 eV 72 ), which is due to the group's chosen oxygen reference, a single oxygen atom in the gas phase, while we use half of the energy of an O 2 molecule. In agreement with Francis and Taylor, we find that hcp adsorption sites are unstable in DFT; an atom placed there relaxes, in the case of Mg(0001), to the tet-1 position.