Modeling a liquid crystal dynamics by atomistic simulation with an ab initio derived force field.

Abstract

Atomistic molecular dynamics (MD) simulations of 4-n-pentyl 4'-cyano-biphenyl (5CB) have been performed, adopting a specific ab initio derived force field. Two state points in the nematic phase and three in the isotropic phase, as determined in a previous work, have been considered. At each state point, at least 10 ns have been produced, allowing us to accurately calculate single-molecule properties. In the isotropic phase, the values of the translational diffusion coefficient, and even more so the activation energy for the process, agree well with experimental data. Qualitatively, also the dynamic anisotropy of the nematic phase is correctly accounted for. Rotational diffusion coefficients, which describe spinning and tumbling motions, fall well within the range of experimental values. The reorientational dynamics of our model 5CB covers diverse time regimes. The longest one is strongly temperature dependent and characterized by a relaxation time in accord with experimental dielectric relaxation data. Shear viscosity and Landau-de Gennes relaxation times, typically collective variables, reproduce the experimental results very well in the isotropic phase. In the nematic phase, despite a large statistical uncertainty due to the extremely slow relaxation of the correlation functions involved, our simulation yields the correct relative order of the three experimental Miesowicz viscosities.