| In recent years, the ground state geometrical structures and the nature of nano-clusters attract researchers'attention more and more, which is different from their macroscopic theories. One of the most interesting aspects is the study of thermodynamic properties of clusters, because it has important scientific significance and potential value. Therefore, focusing on clusters, we carry out a large number of experiments and numerical simulations about it and find a series of peculiar nature on it. For example, some clusters exist surface melting and isomerization in low temperature. As the temperature increases, the various isomers don't transfer continuously until completely melt into liquid. In other words, they are called "pre-melting". In the current conditions, it is difficult to have a direct observation of dynamic cluster at the atomic level through experiments. Therefore, the molecular dynamics simulation becomes the ideal tool to research it .The melting properties of Irn(n=13,14,55,56,147)clusters are studied by molecular dynamics simulations with two different potentials: the Gupta and the Sutton-Chen potential. As for the comparative study of the melting process of Iridium clusters from solid to liquid, the temperature scale is controlled by Berendsen method, and the time step is 1 fs. Started from the ground-state geometrical structures, the system was heated from 50 K to 3000 K with the temperature interval of 10K for Irn (n = 13,14 , 55,56) and 20K for Ir147. At each point of temperature, 5×106 MD steps are integrated for Irn (n = 13,14 , 55,56) clusters and 2×106 steps of MD steps are integrated for Ir147 cluster.Our result indicates that: the same ground state geometrical structures are obtained for both potentials. Variations of melting temperature with cluster size of the two different potentials are consistent with each other, and variations of pre-melting temperature interval with cluster size are exactly the same for both potentials. For the small clusters (Ir13, Ir14), both potentials show that the appearance of the maximum of the heat capacity curve is delayed from the rapid change of the Lindemann index. |
