Structure of 55-atom bimetallic clusters

We studied the structure of bimetallic clusters with 55 atoms having the basic geometry of a Mackay icosahedron. The energy of the clusters is calculated as a sum of pairwise Morse functions, which is a purely covalent term, plus the electrostatic energy of screened atomic charges obtained through the electronegativity equalization (EE) method. The parameters De, Re, and ae of the Morse functions vary according to the atoms' coordination and interpolate smoothly between the known (experimental) diatomic and bulk limits. The EE parameters are fitted to density functional theory ionization energies of small model clusters. The energy of the clusters AnB(55−n) (x=13 and 27, and A,B=Cu,Ag, and Au) was minimized with respect to site occupancy by atoms of each type by combined simulated annealing and conjugate gradient optimization. The lowest energy structures are characterized by: (i) segregation of the metal with lowest surface energy to the surface of the cluster; (ii) occupancy of the atom-capping surface sites (sites with a coordination of 6) by the lowest cohesive energy metal; (iii) a tendency toward AB ordering, to maximize ionic interactions, at the surface of AuAg and AuCu clusters; (iv) a compromise between mixing and surface segregation in some of the CuAu and AuAg clusters.