  Mean square displacements of hydrogen atoms in glass-forming materials and
proteins, as reported by incoherent elastic neutron scattering, show kinks in
their temperature dependence. This crossover, known as the dynamical
transition, connects two approximately linear regimes. It is often assigned to
the dynamical freezing of subsets of molecular modes at the point of equality
between their corresponding relaxation times and the instrumental observation
window. The origin of the dynamical transition in glass-forming glycerol is
studied here by extensive molecular dynamics simulations. We find the dynamical
transition to occur for both the center of mass translations and the molecular
rotations at the same temperature, insensitive to changes of the observation
window. Both the translational and rotational dynamics of glycerol show a
dynamic crossover from the structural to a secondary relaxation at the
temperature of the dynamical transition. A significant and discontinuous
increase in the orientational Kirkwood factor and in the dielectric constant is
observed in the same range of temperatures. We, however, do not find any
indications of a true thermodynamic transition to an ordered low-temperature
phase. We therefore suggest that all observed crossovers are dynamic in
character. The increase in the dielectric constant is related to the dynamic
freezing of dipolar domains on the time-scale of simulations.
