Molecular dynamics simulation study of the high frequency sound waves in the fragile glass former orthoterphenyl

Using a realistic flexible molecule model of the fragile glass formerorthoterphenyl, we calculate via molecular dynamics simulation the collectivedynamic structure factor, recently measured in this system by Inelastic X-rayScattering. The comparison of the simulated and measured dynamic structurefactor, and the study of its properties in an extended momentum, frequency andtemperature range allows: i) to conclude that the utilized molecular modelgives rise to a dynamic structure factor in agreement with the experimentaldata, for those thermodynamic states and momentum values where the latter areavailable; ii) to confirm the existence of a slope discontinuity on theT-dependence of the sound velocity that, at finite Q, takes place at atemperature 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 Qlimit is consistent with T_g. The latter finding is interpreted within theframework of the current description of the dynamics of supercooled liquids interms of exploration of the potential energy landscape.Comment: RevTex, 9 pages, 10 eps figure