For all systems considered, the surface alloys are stable with respect to phase separation (negative H). Most stable is the PtAg/Ag1L/Pt3 system, although the differences between the different systems (PtAg/Pt4L, PtAg/Ag1L/Pt3L, PtAg/Ag2L/Pt2L) are small. In a simple picture, one would expect a higher formation enthalpy for the PtAg/Ag1L/Pt3L system compared to PtAg/Pt4L, based on vertical ligand effects and bond order conservation, which is obviously not the case. Again, we expect this to result from contributions from the Pt-Ag interface(s). Using RPBE/PAW, there are no differences in the general trends (see Table , ESI). There are some differences in individual results, but these are at the limit of the accuracy of the calculations.