Observable-dependence of the effective temperature in off-equilibrium diatomic molecular liquids

We discuss the observable-dependence of the effective temperature $T_{eff}$, defined via the fluctuation-dissipation relation, of an out-of-equilibrium system composed by homonuclear dumbbell molecules. $T_{eff}$ is calculated by evaluating the fluctuation and the response for two observables associated respectively to translational and to rotational degrees of freedom, following a sudden temperature quench. We repeat our calculation for different dumbbell elongations $\zeta$. At high elongations ($\zeta > 0.4$), we find the same $T_{eff}$ for the two observables. At low elongations ($\zeta \leq 0.4$), only for very deep quenches $T_{eff}$ coincides. The observable-dependence of $T_{eff}$ for low elongations and shallow quenches stresses the importance of a strong coupling between orientational and translational variables for a consistent definition of the effective temperature in glassy systems.