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 base s, 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 clus ters with N = 55, 135, and 140 atoms. The N = 55 and 135 clusters were chosen because of simultaneous cube-octahedral (fee) and icosahedral (ices) subshell closings, and we find ices geometries are preferred. R emarkably large compressions of the central atoms are observed for the ices structures (up to 6% compared with bulk interatomic spacings), w hile small core compressions (similar to 1%) are found for the fee geo metry. In contrast, large surface compressive relaxations are found fo r the fee clusters (similar to 2%-3% in average nearest neighbor spaci ng), while the ices surface displays small compressions (similar to 1% ). Energy differences between ices and fee are smallest for Pd, and fo r all systems the single-particle densities of states closely resemble s bulk results. Calculations with N = 134 suggest slow changes in rela tive energy with N. Noting that the 135-atom fee has a much more open surface than the ices, we also compare N = 140 ices and fee, the latte r forming an octahedron with close packed facets. These ices and fee c lusters have identical average coordinations and the octahedron is fou nd to be preferred for Ru and Pd but not for Ag. Finally, we compare H arris functional and LDA energy differences on the N = 140 clusters, a nd find fair agreement only for Ag.