A ONE-PARAMETER MODEL OF A RIGID-UNIT STRUCTURE

The Rigid-Unit model has been developed to describe phase transitions in which polyhedral units of several atoms maintain their integrity on collective motion. Previously, lattice dynamics calculations have bee n performed on such structures at zero temperature and with infinitesi mal rotations of the rigid units. Molecular Dynamics calculations were carried out on the P-Cristobalite structure to observe its behaviour under the excitation of several Rigid Unit Modes (RUMs) at finite temp eratures. Rigid tetrahedra interacted via harmonic forces at the verti ces - the 'one-parameter' model. RUMs are seen, in this model, to free ly coexist with no particular preference for one set of RUMs over anot her.