Furthermore, these data also indicate a weak driving force for 2D clustering in the surface layer, in agreement with experimental findings. As an example, the enthalpy gain obtained for creating a surface with 4 unit cells of Pt1Ag8/Pt4L with a total H of 4 x 9 x -0.26 eV = -9.36 eV is slightly lower than that for creating a surface with 3 unit cells of Ag1L/Pt4L (3 x 9 x -0.30 eV = -8.1 eV) plus one unit cell of Pt1Ag8/Pt4L (9 x -0.15 eV = -1.35 eV), which yields in total -9.46 eV. Similar results are obtained also for other combinations.