Molecular dynamics simulations of the bulk phases of 4-cyano-4′-n-pentyloxybiphenyl

Abstract

Abstract Extensive molecular dynamics (MD) simulations of the crystalline, the isotropic, and the nemadc phase of 4-cyano-4′-npentyloxybiphenyl (50CB) are reported. Basing on the AMBER and the OPLS force field parameters for 5OCB were modified in order to perform the simulations within a model scenario, where the dipole moment and the rotational barriers are in agreement with experimental data. Up to now this is the first investigation of the problem, whether long-range orientational equilibrium can be achieved on the time scale of MD simulations with a full-detail molecular model. The simulations of the liquid-crystalline phase were started from configurations with different initial order parameters. Each order parameter remains stable even over the longest MD run of more than 1.6 ns. Collective motions of molecules and slow self-diffusion are discussed in the context of very slow equilibration of long-range order. In contrast, equilibrium of short-range orientational and positional order is reached. Antiparallel dimers are formed due to strong interactions between two cyano groups. Furthermore, a high probability density for cyano groups adjacent to the phenyl rings of the biphenyl moiety is observed. Although both effects are more pronounced in the liquid-crystalline phase, the differences in short-range order between the liquid and the liquid-crystalline phases are small.