Phase diagram of the system with the repulsive shoulder potential in two   dimensions: density functional approach
  In the framework of the density functional theory of freezing proposed in our previous works, we calculate the phase diagram of two-dimensional system of particles interacting through the repulsive shoulder potential. This potential consists of the hard core and repulsive shoulder of the larger radius. It is shown that at low densities the system melts through the continuous transition in accordance with the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) scenario, while at high densities the conventional first order transition takes place. 
E. S. Chumakov, Yu. D. Fomin, E. L. Shangina, E. E. Tareyeva, E. N.   Tsiok, V. N. Ryzhov
cond-mat.stat-mech