Ab initio calculations of Ru, Pd, and Ag cluster structure with 55, 135, and 140 atoms

A massively parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N=55, 135, and 140 atoms. The N=55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) subshell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions (∼1%) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters (∼2%–3% in average nearest neighbor spacing), while the icos surface displays small compressions (∼1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bu...