Global optimization of bimetallic cluster structures. I. Size-mismatched Ag--Cu, Ag--Ni, and Au--Cu systems

A genetic algorithm approach is applied to the optimization of the potential energy of a wide range of binary metallic nanoclusters, Ag–Cu, Ag–Ni, Au–Cu, Ag–Pd, Ag–Au, and Pd–Pt, modeled by a semiempirical potential. The aim of this work is to single out the driving forces that make different structural motifs the most favorable at different sizes and chemical compositions. Paper I is devoted to the analysis of size-mismatched systems, namely, Ag–Cu, Ag–Ni, and Au–Cu clusters. In Ag–Cu and Ag–Ni clusters, the large size mismatch and the tendency of Ag to segregate at the surface of Cu and Ni lead to the location of core-shell polyicosahedral minimum structures. Particularly stable polyicosahedral clusters are located at size N = 34 ͑at the composition with 27 Ag atoms͒ and N = 38 ͑at the composition with 32 and 30 Ag atoms͒. In Ag–Ni clusters, Ag32Ni13 is also shown to be a good energetic configuration. For Au–Cu clusters, these core-shell polyicosahedra are less common, because size mismatch is not reinforced by a strong tendency to segregation of Au at the surface of Cu, and Au atoms are not well accommodated upon the strained polyicosahedral surface.