Calculation of\nDielectric Spectra by Molecular Dynamics:\n4‑Cyano-4′-hexylbiphenyl in its Nematic Phase

We employ classical molecular dynamics simulations to\npredict the\nexperimentally validated dielectric spectra of a bulk 4-cyano-4′-hexylbiphenyl\n(6CB) system in the nematic phase at temperatures of 300 K and 290\nK. We separately analyze the dielectric spectra parallel to the nematic\ndirector and perpendicular to it. They show different intensities\nand different relaxation times, with the parallel relaxation being\nslower (hundreds of nanoseconds) than the perpendicular (about 1 ns).\nWe investigate various molecular motions as possible mechanisms for\nthe observed macroscopic dielectric behavior by determining their\ncharacteristic time scales. We find that the parallel dielectric relaxation\nis realized by “head-over-heel” flips of 6CB molecules\nthat invert their dipole moment. The perpendicular dielectric relaxation\nis traced to a combination of the precession of the long molecular\naxis (dipole axis) around the director and reorientations of the phenylene\nrings around the long-axis. Dihedral-angle transitions in the <i>n</i>-hexyl tails do not contribute to the dielectric signal.