Order and disorder in molecular crystals: trans-stilbene

Recent refinements of the structure of trans-stilbene suggested the presence of disorder at only one of the two crystallographically independent sites of the unit cell. Calculations based on atom-atom intermolecular potentials have shown that this disorder is adequately described by a model in which some of the molecules at that site are misoriented in a manner which results in large positional changes for only a few of the atoms in each molecule. A technique has been developed for including the degree of misorientation into the lattice energy calculations and the results are compatible both with the preference for disorder at one site and with the degree of misorientation at that site as estimated from difference maps. The system was analysed at two temperatures (113 and 298 K). The calculations confirm that the probability of orientational disordering at low temperature is smaller than at high temperature and show that it is limited to about 20% in the latter case.