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Changes in Gibbs free energies for the process Arn-i,(a) + Ar(a) ͢ Arn,(a) are calculated by Monte Carlo simulations for Ar clusters physisorbed on a graphite substrate. Calculations are performed for clusters Ar2 through Ari2 at 10 K. Using a simulated annealing procedure, the minimum-energy configurations at 0 K are obtained. In all cases studied minimal-energy two-dimensional structures are found at a distance above the surface identical to that determined for an argon monomer. Some cluster sizes exhibit isomerization between several low-energy configurations during the simulations. This isomerization leads to sampling difficulties that are significantly reduced by using the Jwalking method. Minima in the Gibbs free energy are found for cluster sizes 7, 10, and 12. An explanation for the location of the minima in the free-energy function is given in terms of cluster structure and energetics.

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© 1992 American Institute of Physics.



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