ORDERED STRUCTURES IN NUCLEAR AND ATOMIC PHYSICS

With the help of molecular dynamics computer simulations we study the equilibrium configurations and the volume and surface energies at low temperature of large systems of classical interacting particles either under the influence of their mutual long range Coulomb forces and a r adial harmonic external confining force or of the short range Lennard- Jones potential. The former is a model for charged particles in ion tr aps and the latter for clusters. For the Coulomb plus harmonic force, the particles arrange in concentric spherical shells with hexagonal st ructures on the surfaces. The closed shell particle numbers agree well with those of multilayer icosahedra (mli). A Madelung (excess) energy of -0.8926 is extracted which is larger than the bcc value. The resul ts are compared with those obtained by a simple onion shell model. For the Lennard-Jones force we employ various initial configurations like multilayer icosahedra or hexagonal dosed packed (hcp) spheres. Cohesi ve (volume) and surface energies per particle are extracted and compar ed to the energies of scaled mli quasicrystals and of spherical scaled crystals with N up to 36 000. It is shown that relaxed mli are the do minant structures for N < 5 000 and hcp spheres for larger particle nu mbers. For N < 22 000, hcp crystals have about the same closed shell n umbers as mli quasicrystals but smaller ones for N > 22 000. The same magic numbers are obtained with other short range Mie potentials. The stable Argon cluster configurations calculated with a realistic pair p otential are mli for N < 750, hcp for 750 < N < 9300 and face centered cubic (fcc) for N > 9300.