Molecular dynamics studies of the condensed phases of n-butane and their transitions: II. The transition to the true plastic phase

Molecular dynamics (MD) simulations of solid n-butane have been performed with the model described in paper I. All calculations were done on the massively parallel ICL DAP computer which made possible the large number of molecules (2048) and the long simulation runs. The structures of all three experimental phases as observed by neutron diffraction have been closely reproduced by the simulation. The plastic phase structure was not input into the simulation; instead the transition from the metastable phase II was simulated, yielding phase I after transition in very satisfactory agreement with experiment. The experimental phase III is metastable in our model since its energy is 1 per cent higher than the monoclinic structure of paper I. Analysis of the statics and dynamics of the plastic phase show that molecules are disordered about the long molecular axis, and re-orient about that axis between one major and three subsidiary sites. Large-amplitude libration means that the distribution about these sites is broad; the width of the major peak in the orientational distribution function is 40° in reasonable agreement with experiment. A rotational diffusion coefficient D r =0·0716 rad 2 ps -1 and correlation time τ = 0·0363 ps were obtained.