System of particles with interaction and the cluster formation in   condensed matter
  We investigate the behaviour of a system of particles with the different character of interaction. The approach makes it possible to describe systems of interacting particles by statistical methods taking into account a spatial nonhomogeneous distribution of particles, i.e. cluster formation. For these clusters are evaluated: their size, the number of particles in a cluster, and the temperature of phase transition to the cluster state. Three systems are under consideration: electrons on the liquid helium surface, particles interacting by the shielding Coulomb potential, which are found under the influence of an elastic field (e.g. nucleons in a nucleus), and gravitating masses with the Hubble expansion. 
Volodymyr Krasnoholovets and Bohdan Lev