Theoretical studies of the structure and dynamics of metal/hydrogen systems: Diffusion and path integral Monte Carlo investigations of nickel and palladium clusters

Abstract

Using both classical and quantum mechanical Monte Carlo methods, a number of properties are investigated for a single hydrogen atom adsorbed on palladium and nickel clusters. In particular, the geometries, the preferred binding sites, site specific hydrogen normal mode frequencies, and finite temperature effects in clusters from two to ten metal atoms are examined. Our studies indicate that hydrogen is localized in the present systems. The preferred hydrogen binding sites are found to be tetrahedral in clusters with five or fewer metal atoms and octahedral for clusters of six to ten atoms. The exceptions to this rule are Ni 9 H and Pd 9 H for which the outside, threefold hollow and the inside tetrahedral sites are preferred, respectively. Hydrogen induced ''reconstruction'' of bare cluster geometries is seen in seven and ten-atom clusters.

A semi-empirical potential for simulations of transition metal clusters: Minima and isomers of Nin (n=2–13) and their hydrides

“…1. Unlike recent findings 5 with the embedded atom model, we do not find any H atoms in the tetrahedral site. The tetrahedral configuration of Ni 4 H should distort as consequence of an orbital degeneracy.…”

“…The lowest minimum for Ni 6 proposed by the authors of this study is the octahedron in agreement with the results of both previous CEM 8 and EAM models. 5 The existence of the capped square pyramid has been hypothesized by Parks et al 29 as a possible rearranged structure when one additional N 2 molecule is accommodated at elevated N 2 pressures.…”

“…Previous studies of nickel clusters have used both these methods. [5][6][7][8][9] While semiempirical methods provide potentials that are by no means exact, physical effects characteristic of a class of systems are observable that cannot be seen with alternate representations of the interatomic potential. Examples of these effects are discussed in the current work.…”

“…With these models it is possible to carry out dynamical and statistical simulations with reasonable amounts of computational resources. For transition metal clusters, most simulations [5][6][7] to date have used less expensive potential functions. Examples include the embedded atom model ͑EAM͒ [5][6][7] and the corrected effective medium theory ͑CEM͒.…”

“…Examples include the embedded atom model ͑EAM͒ [5][6][7] and the corrected effective medium theory ͑CEM͒. 8,9 In the embedded atom model 5 the overall potential energy is constructed from spherically symmetric charge distributions about each atom.…”