Tight-binding study of relaxation in Rh-N and Pd-N clusters (9 <= N <= 165)

We have used a tight-binding total-energy expression to study the electroni c, magnetic, and structural properties of relatively large Rh-N and Pd-N cl usters (9 less than or equal to N less than or equal to 165). An important feature of our approach is that it treats simultaneously, and in a consiste nt way, both atomic and electronic structures by performing a relaxation pr ocess using molecular dynamics based on the method of Car and Parrinello. T he clusters are built by adding successive atomic shells around a central a tom. These shells are allowed to relax independently and obey symmetry cons traints. It is found that although an overall contraction occurs, the relax ed structure is far from being homothetical to the unrelaxed one. For small RhN clusters (N less than or equal to 75) multiple magnetic solutions are obtained and in all cases the local magnetic moments M(i) present a remarka ble size and environment dependence. The relation between the observed aver age magnetization and the cluster geometry is also analyzed. Perspectives o f extensions of this study are discussed.