ROTATIONAL-DYNAMICS IN ORTHO-TERPHENYL - A MICROSCOPIC VIEW

Molecular dynamics simulations of a simple model for ortho-terphenyl a re used to investigate the microscopic nature of rotational dynamics i n the supercooled regime. A detailed analysis of the van Hove correlat ion function, as well as direct examination of the orientational motio n of selected molecules, show that, at sufficiently low temperatures a nd on the nanosecond timescale, orientational motion takes place in th e near absence of translational motion. The nature of the orientationa l motion covers a large spectrum of situations, but we find a preponde rance of rapid reorientations, i.e., jumps in two- (or multi-) level s ystems. It is noted, in addition, that the observed relaxation does no t manifest itself as a peak in such quantities as the intermediate sca ttering function and the mean-square displacements. The feature someti mes observed (as in the simulations carried out by the present authors ) in these functions is merely an artifact of the model coming from th e use of a finite-size molecular dynamics supercell.