In the low temperature lattices, the FCC (alpha) and body-centered tetragonal (gamma) lattices), the molecules are orientationally ordered at low temperatures as shown below, and disordered at high temperatures. The HCP (beta) lattice is orientationally disordered at all temperatures, and precesses about the "c" crystal axis. Experimentally both the alpha and gamma lattices are observed to become disordered as the temperature approaches that of the phase transition. In our simulation energies depend solely on orientation of the molecules, so the increase in disorder is reflected in the increase of lattice energy with increase in temperature [2].
 
 

Fig 2. The FCC (alpha) lattice (from Mulder et al.)

 

Fig. 3  The HCP (beta) lattice (left, from Mulder et al.) and precession of single molecule in the lattice lattice (right, from Scott).

 

Fig. 4  Gamma lattice (from Mulder et al.)

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