  We investigate in this paper the ground state and the nature of the
transition from an orientational ordered phase at low temperature to the
disordered state at high temperature in a molecular crystal. Our model is a
Potts model which takes into account the exchange interaction $J$ between
nearest-neighbor molecules and a dipolar interaction between molecular axes in
three dimensions. The dipolar interaction is characterized by two parameters:
its amplitude $D$ and the cutoff distance $r_c$. If the molecular axis at a
lattice site has three orientations, say the $x$, $y$ or $z$ axes, then when
D=0, the system is equivalent to the 3-state Potts model: the transition to the
disordered phase is known to be of first order. When $D\neq 0$, the
ground-state configuration is shown to be composed of two independent
interpenetrating layered subsystems which form a sandwich whose periodicity
depends on $D$ and $r_c$. We show by extensive Monte Carlo simulation with a
histogram method that the phase transition remains of first order at relatively
large values of $r_c$.
