  Using a realistic flexible molecule model of the fragile glass former
orthoterphenyl, we calculate via molecular dynamics simulation the collective
dynamic structure factor, recently measured in this system by Inelastic X-ray
Scattering. The comparison of the simulated and measured dynamic structure
factor, and the study of its properties in an extended momentum, frequency and
temperature range allows: i) to conclude that the utilized molecular model
gives rise to a dynamic structure factor in agreement with the experimental
data, for those thermodynamic states and momentum values where the latter are
available; ii) to confirm the existence of a slope discontinuity on the
T-dependence of the sound velocity that, at finite Q, takes place at a
temperature T_x higher than the calorimetric glass transition temperature T_g;
iii) to find that the values of T_x is Q-dependent and that its vanishing Q
limit is consistent with T_g. The latter finding is interpreted within the
framework of the current description of the dynamics of supercooled liquids in
terms of exploration of the potential energy landscape.
