  Molecular dynamics simulations (mds) were carried out to investigate the
reorientational motion of a rigid (fixed bond length), asymmetric diatomic
molecule in the liquid and glassy states. In the latter the molecule reorients
via large-angle jumps, which we identify with the Johari-Goldstein (JG)
dynamics. This relaxation process has a broad distribution of relaxation times,
and at least deeply in the glass state, the mobility of a given molecule
remains fixed over time; that is, there is no dynamic exchange among molecules.
Interestingly, the JG relaxation time for a molecule does not depend on the
local density, although the non-ergodicity factor is weakly correlated with the
packing efficiency of neighboring molecules. In the liquid state the frequency
of the JG process increases significantly, eventually subsuming the slower
alpha-relaxation. This evolution of the JG-motion into structural relaxation
underlies the correlation of many properties of the JG- and alpha-dynamics.
