Formation of Polymorphic Cluster Phases for Purely Repulsive Soft   Spheres
  We present results from density functional theory and computer simulations that unambiguously predict the occurrence of first-order freezing transitions for a large class of ultrasoft model systems into cluster crystals. The clusters consist of fully overlapping particles and arise without the existence of attractive forces. The number of particles participating in a cluster scales linearly with density, therefore the crystals feature density-independent lattice constants. Clustering is accompanied by polymorphic bcc-fcc transitions, with fcc being the stable phase at high densities. 
Bianca M. Mladek, Dieter Gottwald, Gerhard Kahl, Martin Neumann,   Christos N. Likos