The differences between the two shapes in the average coordination number for Au and Pt atoms, ∆ CN = CN ATO Au -CN TO Au are shown in Fig. and Fig. . The most prominent feature in the evolution of ∆ CN lies precisely in the yellow-shaded region, where the Au and Pt in the truncated octahedron are significantly overand under-coordinated, respectively, with respect to the distorted shape. The larger core of the distorted shape therefore allows to over-coordinate Pt at the expense of under-coordinating Au. For concentrations n Au > 96, all surface sites are occupied by Au, the average coordination number of Au and Pt are the same for the two shapes, and the truncated octahedron is again the most stable shape. We note the presence of another feature at (20 ≤ n Au ≤ 30). This emerges because Au occupies all (24) corners of the TO first, whereas it is more favorable to occupy some edge positions of the distorted shape before filling all of its corners.