Molecular dynamics of the rough sphere fluid. I. Rotational relaxation

A systematic study is made of rotational relaxation in rough sphere fluids for densities ranging from 0.1 to 0.666 of the closest packed density. Many of the current models of rotational relaxation are tested against the results of molecular dynamics. It is found that for densities higher than 0.40 of the closest packed density only the Fokker–Planck model is consistent with experiment. The various extended diffusion models, the jump diffusion model, and the rotational diffusion model are not only internally inconsistent, but also do not agree with experiment. No theory is consistent with experiment at intermediate densities where the collisional and the inertial time scales are comparable. In addition to the foregoing, it appears that the rough spheres tumble more freely in the solid state than they do in the fluid state at the melting density.

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