Molecular-dynamics study of supercooled ortho-terphenyl

We present the results of a detailed molecular-dynamics study of relaxation in the van der Waals system ortho-terphenyl in the supercooled regime. The molecule is described by a simple rigid three-site model, with interactions between different molecules of the Lennard-Jones form. We find that the long-time (α) relaxation, as determined from the intermediate scattering function, is well described by a Kohlrausch law. The zero-time amplitude of this process, often referred to as the ‘‘nonergodicity parameter,’’ which can be interpreted as a Debye-Waller factor, is seen to depart from a linear temperature dependence and seems to exhibit a ‘‘singularity’’ at a temperature substantially larger than the conventional glass transition temperature, in accord with neutron-scattering data, and as predicted by mode-coupling theory. We find that the observed anharmonic behavior around ${\mathit{T}}_{\mathit{c}}$, as well as the decay of the scattering functions, is equally evident from both orientational and translational correlations. The latter observation indicates that the structural changes that take place during α relaxation are neither specifically orientational nor specifically translational. Investigation of the van Hove self-correlation for both center-of-mass and orientational motion reveals the existence of a strong non-Gaussian spatial dependence at intermediate times and of processes, even at low temperatures, that cannot be described as simple vibrational motion, and which consist of rapid reorientations of the molecules about their essentially frozen centers of mass. This motion may be related to the fast secondary mode seen in experiments.