  An overview of results of models of coupled quantum rotors is presented. We
focus on rotors with dipolar and quadrupolar potentials in two and three
dimensions, potentials which correspond to approximate descriptions of real
molecules adsorbed on surfaces and in the solid phase. Particular emphasis is
placed on the anomalous reentrant phase transition which occurs in both two and
three-dimensional systems. The anomalous behaviour of the entropy, which
accompanies the reentrant phase transition, is also analyzed and is shown to be
present regardless if a phase transition is present or not. Finally, the
effects of the crystal field on the phase diagrams are also investigated. In
two-dimensions the crystal field causes the disappearance of the phase
transition, and ordering takes place via a continuous increase in the value of
the order parameter. This is also true in three dimensions for the dipolar
potential. For the quadrupolar potential in three dimensions turning on the
crystal field leads to the appearance of critical points where the phase
transition ceases, and ordering occurs via a continuous increase in the order
parameter. As the crystal field is increased the range of the coupling constant
over which metastable states are found decreases.
