THE VAPOR-PRESSURE OF GLASSY CRYSTALS OF DIMERS

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.