  We report Monte Carlo simulations that show closed-loop liquid-vapor
equilibrium in a pure substance. As far as we know, this is the first time that
such a topology of the phase diagram has been found for one-component systems.
  This finding has been achieved on a two-dimensional lattice model for patchy
particles that can form network fluids. We have considered related models with
a slightly different patch distribution in the order to understand the features
of the distribution of patches on the surface of the particles that make
possible the presence of the closed-loop liquid-vapor equilibrium, and its
relation with the phase diagram containing the so-called empty liquids.
  Finally we discuss the likelihood of finding the closed-loop liquid-vapor
equilibria on related models for three dimensional models of patchy particles
in the continuum, and speculate on the possible relationship between the
mechanism behind the closed-loop liquid vapor equilibrium of our simple lattice
model and the salt-induced reentrant condensation found in complex systems.
