  We consider cold polar molecules confined in a helical optical lattice
similar to those used in holographic microfabrication. An external electric
field polarizes molecules along the axis of the helix. The large-distance
inter-molecular dipolar interaction is attractive but the short-scale
interaction is repulsive due to geometric constraints and thus prevents
collapse. The interaction strength depends on the electric field. We show that
a zero-temperature second-order liquid-gas transition occurs at a critical
field. It can be observed under experimentally accessible conditions.
