A simulation study of the disordered phase of CBr4. II. Collective properties and rotation-translation coupling

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Abstract
For pt.I see ibid., vol.19, p.3325 (1986). Molecular dynamics simulation techniques have been used to study the orientationally disordered phase of carbon tetrabromide at 345K, with particular emphasis placed on analysing the coupling between orientations and translations. The wave-vector dependence of this coupling has been calculated, showing that there are no couplings of any sort at k=0, and that over the surface of the Brillouin zone boundary there is a strong coupling between the transverse acoustic modes and the orientational variables that order in the low-temperature phase. The phase transition is discussed within the context of this coupling. The collective dynamics have been studied by calculations of various correlation functions, which become long-lived when the orientational-translation coupling becomes strong. It is shown that a simple Mori theory that includes this coupling is able to reproduce the observed behaviour. In particular, it has been found that when the coupling is strong, the acoustic modes appear in a coherent inelastic neutron scattering experiment as quasielastic scattering with full width at half-maximum height of less than 0.03 THz.