Structural and magnetic isomers of small Pd and Rh clusters:: an ab initio density functional study

We present a comprehensive investigation of the structural, electronic, and magnetic properties of Pd-N and Rh-N clusters with up to N = 13 atoms, based on ab initio density functional calculations. The novel aspects of our investigation are the following. (i) The structural optimization of the cluster by a symmetry-unconstrained static total-energy minimization has been supplemented for larger clusters (N >= 7) with a search for the ground state structure by dynamical simulated annealing. The dynamical structural optimization has led to the discovery of highly unexpected ground state configurations. (ii) The spin-polarized calculations were performed in a fixed-moment mode. This allowed us to study coexisting magnetic isomers and led to a deeper insight into the importance of magnetostructural effects. For both Pd and Rh the larger clusters adopt ground state structures that can be considered as fragments of the face-centred cubic crystal structure of the bulk phase. For Pd clusters, the magnetic ground state is a spin triplet (S = 1) for N <= 9, a spin quintuplet (S = 2) for N = 10, and a spin septet (S = 3) for the largest clusters. Large magnetic moments with up to S = 8 have been found for Rh clusters. Magnetic energy differences have been found to be small, such that there is an appreciable probability of. finding excited magnetic isomers even at ambient temperatures. in several cases, the structural energy differences are also sufficiently small to permit the coexistence of polytypes.