  The phase diagram of a simple model with two patches of type A and ten
patches of type B (2A10B) on the face centred cubic lattice has been calculated
by simulations and theory. Assuming that there is no interaction between the B
patches the behavior of the system can be described in terms of the ratio of
the AB and AA interactions, r. Our results show that, similarly to what happens
for related off-lattice and two-dimensional lattice models, the liquid-vapor
phase equilibria exhibits reentrant behavior for some values of the interaction
parameters. However, for the model studied here the liquid-vapor phase
equilibria occurs for values of r lower than 1/3, a threshold value which was
previously thought to be universal for 2AnB models. In addition, the theory
predicts that below r = 1/3 (and above a new condensation threshold which is <
1/3) the reentrant liquid-vapor equilibria is so extreme that it exhibits a
closed loop with a lower critical point, a very unusual behavior in
single-component systems. An order-disorder transition is also observed at
higher densities than the liquid-vapor equilibria, which shows that the
liquid-vapor reentrancy occurs in an equilibrium region of the phase diagram.
These findings may have implications in the understanding of the condensation
of dipolar hard spheres given the analogy between that system and the 2AnB
models considered here.
