In a glassy crystal, where the molecules cannot rotate, a question ari
ses as to whether the frozen in rotational entropy should, or should n
ot, be included in the free energy for the purpose of determining the
stability of the crystal relative to another phase. We study fluids an
d crystals of two- and three-dimensional dimers, compare the location
of the observed coexistence lines with the location predicted by each
of the two possibilities and conclude that the equilibrium vapour pres
sure of the glassy crystal is correctly predicted when the rotational
entropy is included in the free energy of the glassy crystals.