Some static, dynamic and thermodynamic properties of clusters

We have developped some theoretical and numerical methods to study the structure, thermodynamics and dynamics of atomic and molecular clusters. The multiple histogram method has been extended to the microcanonical ensemble. We have shown that the LJ clusters made of double icosahedron units display the phenomenon of surface melting. The hierarchy of repartition of isomers on the potential-energy surfaces is evidenced on the multiple phase transitions exhibited by the (NaF)_n clusters. We have also developped a preferential sampling methods to accelerate the convergence of Monte Carlo simulations of free or solvated alkali clusters. By explicitely calculating the microcanonical density of states of an isolated cluster, we have shown that it is possible to include rigorously the conservation of angular momentum in Monte Carlo simulations. We have tested the Monte Carlo growth method for nitrogen molecular clusters. It turns to be poorly efficient for these clusters, because a structural transition between polyicosahedra and multilayer icosahedra occurs at a size of 42 molecules. By investigating a Lennard-Jones model with a three-body interaction potential, we interpret this difference in the crossover point as the result of a strong anisotropy of the molecular potential. We have also checked the relevance of Liapunov exponents in the study of phase transitions in clusters. We have shown that coexistence induced the continuity of the variations of these exponents with internai energy. Finajly, we have studied the possibility of cluster dimerization by entropie effects. At zero temperature and with no proper rotation, a compact triaxial shape is the only structure of Na₁₆. However, (Ar₁₃ )₂ presents a local minimum at long distance, which remains stable up to a temperature close to 25 K. We managed to create this isomer during simulations of collisions with a finite angular momentum. (author)