Generalized van der Waals theory of liquid-liquid phase transitions
  In the framework of the thermodynamic perturbation theory for fluids we study how the phase diagram of an isotropic repulsive soft-core attractive potential, where a liquid-liquid phase transition exists in addition to the standard gas-liquid phase transition, changes by varying the parameters of the potential. We show that existence of the liquid-liquid transition is determined by the interplay of the parameters of the potential and the structure of a reference liquid. 
Yu. D. Fomin, V. N. Ryzhov, E. E. Tareyeva