THERMODYNAMICAL STUDY OF THE INTERACTION BETWEEN CLUSTERS

We investigate the thermodynamical stability of the interaction betwee n two clusters at thermal equilibrium using classical molecular-dynami cs (MD) and Monte Carlo (MC) simulations. The intercluster distance Z is fixed as a parameter in the microcanonical and canonical ensembles. We use and develop several techniques to calculate the fundamental qu antities of interest in these ensembles, namely, the density of states Omega(E,Z) and the partition function Q(T,Z), yielding respectively, the microcanonical entropy S(E,Z) and the Helmholtz free energy F(T,Z) . The multiple histogram method is used to estimate the variations of S with E and of F with T, both extracted from either constant energy M D or constant-temperature MC simulations. The thermodynamic perturbati on and the displacement-vector methods are used to provide the variati ons of the free energy along the Z coordinate. These methods are appli ed to the interaction of Ar-13+Ar-13 and Na-8+Na-8 clusters. The Lenna rd-Jones (Ar-13)(2) cluster dimer has a locally mechanically stable st ructure at 2=8.6 Angstrom, which appears to remain thermodynamically s table until T similar or equal to 25 K. The temperature effects also s tabilize two intermediate compact configurations, near Z=5 and 6.6 Ang strom. On the other hand, the interaction between Na-8+Na-8, modeled b y a distance-dependent tight-binding Hamiltonian, does not exhibit a s table structure except in its compact shape Na-16. The entropic effect s, favored by the thermal phenomena, do not occur to induce any thermo dynamical local stability for a dimerized (Na-8)(2). Ln other terms, t he stability of Na-16 does not seem to be governed by the underlying t wo Nas magic-number units.