Table . Mean enthalpies of formation in eV per surface atom (upper lines), differential enthalpies of formation in eV per surface atom (middle lines), and surface energies in meV A -2 (bottom lines), for the PtAg/AgnL/Pt(111) surface alloys indicated. 1 meV A -2 = 16.0210 J m -2 . For the systems with a Pt(111) lattice, 1 meV per surface atom corresponds to 0.1462 meV Å -2 for a slab of identical composition/thickness. Results were obtained using PBE/PAW, data using RPBE/PAW are collected in Table As shown in Table and in Figure , the formation enthalpy decreases almost linearly (becomes less negative) with increasing Pt content in the topmost layer for PtAg/Pt4L. Conversely, it increases with increasing Ag contents. In Figure , the first (x=0) and last (x=1) point represent the values for the Pt(111) and Ag1LPt4L systems, respectively. The approximately linear decay of the mean formation enthalpy per surface atom can be converted into a differential value, indicating the differential change in system energy upon exchanging another surface atom, which are also listed in Table . Here we see a more pronounced decay, reaching the zero level at n=4. Consequences of these data with respect to the preferential formation of larger ensembles will be discussed later in this section. Finally, the surface energy increases with increasing Pt content in the surface layer, due to the stronger Pt -Pt bonds that have to be broken upon cleaving the crystal.