Finally, we would also like to comment on the tendency for two-dimensional phase separation in the surface layer of the surface, which was indicated by scanning tunneling microscopy data for PtAg/Pt(111) monolayer surface alloys. Comparing the slab formation energies of -39.35 meV A - ² for the Ag1L system and -26.82 meV A -² for the Pt1Ag3/Pt(111) surface alloy (see Table ), we get mean formation energies of 3 /4  -39.35 = -29.5 meV A -² for the phase separated Pt1Ag3/Pt(111) system which is favorable compared to the formation energy of -26.82 meV A -² of the mixed surface layer (Pt@Ag1L), in good qualitative agreement with the experimental observation. In contrast, for Pd1Ag3/Pd(111) surface alloys, which show a random distribution of the Pd and Ag surface atoms, 8 a similar calculation results in (mean) formation energies of 3 /4  -39.57 = -29.7 meV A -² for the phase separated surface and -32.50 meV A -² for the mixed phase, which favors 2D intermixing, in good agreement with experimental observations. 8