Melting of palladium clusters - Canonical and microcanonical Monte Carlo simulation

We present Monte Carlo simulations of single palladium clusters of 13, 34, 54, 55, 147 and 309 atoms. The clusters are modeled by a many-body potential and they have been simulated at constant temperature or constant total energy. The caloric curves of the clusters, with the exception of Pd-34, exhibit an S-bend at melting which is typical for a finite system. We have also observed the typical coexistence region of solid and molten clusters both in the canonical and the microcanonical ensembles. Pd-34, in contrast, melts without an accompanying peak in heat capacity and at melting the atoms become mobile without any significant change in geometric structure. For the larger clusters a free energy barrier inhibits phase switching. In some cases of phase change from molten to solid structure the barrier is of purely entropic character. By a conversion of the results in the microcanonical simulations into temperature-dependent data, the simulations at fixed temperature and fixed total energy have been compared. The agreement is in most cases good. The results are furthermore compared to earlier molecular dynamics simulations with the Nose-Hoover thermostat. These results are in good agreement with the Monte Carlo simulations as well.