Molecular Dynamics Calculations Of Melting and Fragmentation of Ar and Ag Clusters
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However, according to early capillarity theories, MD simulations on Ar and Au clusters of size a,-200 indicate that melting starts at the surface, with a consistent desorption of atoms, and a solid phase remains into the core of the melted structure. Consequently, the transition from solid to liquid is continuous and not first-order like (see [2, 3].In [2] all the essential references on melting of Ar clusters are reported). On the other side, Collision Induced Fragmentation (CIF) has been used for more than thirty years to analyze the structure of organic ions and today is applied to clusters of semiconductor,metallic and rare-gas atoms and to fullerenes. Though it was very early pointed out that CIF can be understood within the frame of the evaporative decay due to a temperature increase, this suggestion was not further developed[4]. On the contrary, many theoretical models are based on the similarity between CIF and nuclear collisions. The purpose of this work is to further develop the study on melting and fragmentation by seeking effects related to the cluster size and to the type of bonding for clusters of a more realistic size than in traditional melting studies. Furthermore, an attempt is made to state relationships between melting and CIF by comparing the mode of fragmentation in the two cases. The cluster components are Ar and Ag and the cluster size N falls in the range 30500.The analysis of the cluster evolution is made by using Molecular Dynamics with classical forces. 661 Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
The computational model. Our calculations consist of two stages, one of equilibration, to find a stable structure at the energy minimum, and one of dynamical evolution under the effects of the given energy input. Both phases are calculated in the canonical ensemble,at constant temperature, by using the Andersen method. The configuration at the beginning of the equilibration phase is a fcc crystalline fragment.The stability of the stationary state is tested in a MD run, without any input energy, of a duration of time around 800 ps. After equilibration, to simulate the effects of a heat source, the central value of the velocity distribution is suddenly changed to the wanted temperature T. Otherwise, in the CIF mode, to simulate the increase of energy due to an impact collision with another cluster, the kinetic energy of a given atom is raised to the wanted value. Similarly to nuclear collisions,this atom is termed Primary Knocked-on Atom(PKA).In the following,for the sake of brevity, the two cases are indicated as 'temperature'or 'collisional' transient,respectively. The fraction of PKAs tested in a 'collision' calculation has been varied from 50% in the smaller clusters to 20,30% in the larger ones. The energy of the PKA varies between 0.1 and 10 eV. The standard Lennard-Jones and the EAM OhJohnson potential are used for Ar and Ag,respectively.This last choice is motivated by the realistic results reported in [1], where a similar EAM potential has been used in MD si
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