  We evaluate the location of the gas-liquid coexistence line and of the
associated critical point for the primitive model for water (PMW), introduced
by Kolafa and Nezbeda [J. Kolafa and I. Nezbeda, Mol. Phys. 61, 161 (1987)].
Besides being a simple model for a molecular network forming liquid, the PMW is
representative of patchy proteins and novel colloidal particles interacting
with localized directional short-range attractions. We show that the gas-liquid
phase separation is metastable, i.e. it takes place in the region of the phase
diagram where the crystal phase is thermodynamically favored, as in the case of
articles interacting via short-range attractive spherical potentials.
Differently from spherical potentials, we do not observe crystallization close
to the critical point. The region of gas-liquid instability of this patchy
model is significantly reduced as compared to equivalent models of spherically
interacting particles, confirming the possibility of observing kinetic arrest
in an homogeneous sample driven by bonding as opposed to packing.
