HIGH-FREQUENCY RELAXATION OF O-TERPHENYL

Results of molecular dynamics simulations (MDS) of o-terphenyl, a glass-forming liquid, are analyzed in terms of the coupling model of relaxation. At low temperatures thermally activated relaxation processes are suppressed, whereby the density-density correlation function, C(t), obtained by MDS is determined entirely by vibrational modes. This enables the low temperature data to be used to deduce the vibrational density of states, g(omega). With g(omega) determined, the vibrational contribution, C-pho(t), is calculated at higher temperatures assuming that g(omega) is independent of temperature. At higher temperatures, relaxation makes its appearance and is modeled here by the fast dynamics of the coupling model. Assuming that vibration and relaxation contribute independently, the density-density self-correlation function is given by the product C-pho(t)C-rel(t), with the relaxation part obtained from the coupling model. There is good overall agreement between the calculated C(t) and the MDS data. Microscopic parameters, including the energy barrier for reorientation of the o-terphenyl molecule, are extracted from the MDS results. (C) 1995 American Institute od Physics.