The structure, chemical composition as well as physical and chemical properties of multi-component solid surfaces are often dominated by their surface energy and formation energy. For instance, the wetting properties of surfaces or the thermodynamic growth modes of thin films 1;2 are determined by the surface and interface energies of substrate and deposit. Unfortunately, the quantitative determination of these parameters is experimentally challenging, and also theoretically, there are problems, at least for a first-principles atomic scale determination. Using periodic density functional theory (DFT) based calculations, surface energies of solids have been commonly determined by cleaving a bulk crystal, which can be calculated by subtracting the bulk energy of the solid from the energy of a limited slab with a defined surface area. Strictly speaking, this requires, that the two new surfaces are identical also in details. In most DFT calculations, slabs are modeled by a few atomic layers and relaxed only in the topmost layers, while in the bottom layers, the atoms are frozen at their bulk positions. In that case, the calculated surface energy is an average from the relaxed and unrelaxed slab surfaces. The averaged value is still close to the surface energy for a monoatomic slab with a small energy gain by the surface relaxation but not for a multicomponent slab with two surfaces consisting of different constituents. To correct for this, the slab needs to be sufficiently thick that both new surfaces can be relaxed and nevertheless contain a bulk-like part in the center.