where Eslab is the total energy of the slab, ni denotes the number of atoms of type i in the slab (per unit cell) and Ei the bulk energies (per atom) of these species. However, surface energies calculated this way represent an average of the surface energies of the upper and lower side of the slab, which are not necessarily the same in our study. Furthermore, these surface energies also contain contributions from the energy of the internal interface between Pt and Ag (interface energy). To avoid the first problem, we will also provide estimated surface energies of the Ag containing surfaces, which were calculated from the value calculated via equation (2) by assuming that the surface energy of the bottom side is identical with that of a Pt(111) surface, which seems to be reasonable. Nevertheless, we will base our discussion of the stability of the different surface configurations mainly on the formation enthalpies and use surface energies only for comparison of rather similar surfaces, such as the Ag film covered surfaces. We would also like to note that while both surface energy and formation enthalpy represent measures of the stability, the references are different. The surface energy, often defined as the energy per surface area required to cleave an infinite crystal along a certain orientation, reflects the driving force towards reducing the surface area, e.g., by sintering, and also the tendency of a given component to accumulate at the surface, e.g., by surface segregation. The formation enthalpy indicates the enthalpy gained upon formation of the respective slab, as compared to the bulk energies of the separate components. Hence, it also indicates the tendency for phase separation. By and large, these trends are also reflected by the surface energies, with higher surface energies for less negative formation enthalpies. Among the Ag containing surfaces, those with Ag at the surface exhibit the lower surface energy. Hence, as expected, the termination by Ag (plus the formation of the Pt-Ag interface) is more favorable than Pt termination. Note that the estimated surface energies of the top layer of the Ag-terminated surfaces differ of theses only little from that of Ag(111). On the other hand, contributions from interface energies are likely responsible for the high value of the surface energy of Pt2L/Ag1L/Pt2L, which is even higher than that of Pt(111).