  The dynamic orientational order-disorder transition of clusters consisting of
octahedral molecules is formulated in terms of symmetry-adapted rotator
functions.
  The transition from a higher-temperature body-centered-cubic, orientationally
disordered phase to a lower-temperature, monoclinic, orientationally-ordered
phase is a two-step process. The higher-temperature transition has two local
minima in the free energy, like a finite-system counterpart of a first-order
transition. Simulations show the lower-temperature transition is continuous.
The analytic theory predicts a temperature of 27K for this transition, for a
59-molecule cluster of tellurium hexafluoride in a good agreement with the ~30K
result of canonical Monte Carlo calculations.
