ENERGIES OF GROUND STATES AND STRUCTURES OF (Ag)_x(Rh)_(n-x) CLUSTERS STUDIED BY A GENETIC ALGORITHM

The tight binding second moment approximation has been employed, in combination with a genetic algorithm, to determine the ground state atomic configrations of Ag x Rh n-x ( n =13,55) clusters for all concentrations x . It is found that the lowest energy structures of both the bimetallic and the pure ( x=0,n ) clusters are slightly distorted with respect to the perfect icosahedron, and the cohesive energies of the ground states increase with increasing the component of Rh. In general, there is a tendency for Ag atoms to be segregated at the surfaces of the bimetallic clusters. At the surfaces of the Ag x Rh 13-x clusters, the Ag atoms and the Rh atoms are separated, and the clusters don't exhibit any ordering. However, at the surfaces of the Ag x Rh 55- x clusters, the Ag atoms and the Rh atoms occupy the positions with high symmetry, and the clusters exhibit ordering.