Unfortunately, the quantitative determination of surface energies is not only experimentally challenging, but also theoretically, at least in an atomistic picture, e.g., by using periodic density functional theory (DFT) based calculations . Surface energies of solids have commonly been calculated as the cleavage energy of a bulk crystal along a given plane, by subtracting the bulk energy of the solid from the energy of a limited slab with a defined surface area A and the same number of atoms per unit cell, Eslab, via The calculation of surface energies requires the use of thick slab models as both sides of the slab need to be relaxed, whereas asymmetric slab models (see Figure ) yield an average surface energy of a relaxed and a frozen surface structure. However, there is also a procedure how from such an asymmetric calculations the surface energy of the relaxed surface can be derived ]. In the case of compounds or bi-or multimetallic systems, the situation is even more complex, as illustrated in Figure for a system consisting of a substrate covered by a film of one or few layers thickness. In this case, the surface energies at the top surface and at the bottom surface are likely to differ significantly, and calculations according to equation ( ) would yield an average value of the two surfaces. Furthermore, since the surface energy of a given surface will depend on the nature of the bonds that have to be broken, it will depend on the nature and structure of the layers on the other side of the intended cleavage planes. This had been discussed in detail in Ref. . As described there, the initial crystal should most simply be symmetric with respect to the cleavage plane, both structurally and with respect to the chemical composition of the respective layers on both sides of the cleavage plane. Such kinds of model systems, which would be consistent also with the situation in an elemental system, are, however, more complex than the model systems used in standard DFT calculations, and have to be constructed for the given system. As an example, in such approach the surface energy of the Ag-terminated surface of a pseudomorphic monolayer film Ag on a bulk substrate Pt , which is modelled by 5 Pt layers (Ag1L/Pt5L), would be calculated as the cleavage energy of a mirror-symmetric slab with 10 Pt layers in the center, followed by 1 pseudomorphic layer of Ag in each direction, where the cleavage plane is above (below) the Ag monolayer (see Figure ). In a similar way, the surface energy of the (bottom) surface of the n-layer Pt( ) substrate (Pt-terminated slab) can be calculated, which for a reasonable thickness of the substrate model should quickly approach the surface energy of pure Pt(111) (for details see ).