Structural relaxation in a supercooled molecular liquid

– We perform molecular-dynamics simulations of a molecular system in supercooled states for different values of inertia parameters to provide evidence that the long-time dynamics depends only on the equilibrium structure. This observation is consistent with the prediction of the mode-coupling theory for the glass transition and with the hypothesis that the potential energylandscape controls the slow dynamics. We also find that dynamical properties at intermediate wavenumber depend on the spatial correlation of the molecule’s geometrical center. Dynamics in normal liquid states deals with orbits in phase space. Binary collisions and vibrations, for which the kinetic energy plays an important role, are elementary ingredients building the motion on microscopic time and frequency scales. In supercooled states, the system gets trapped in potential energy-landscape pockets for long times. This produces a separation of the long-time glassy dynamics — corresponding to the interbasin dynamics, i.e., to the motion from one pocket to the other — from the microscopic dynamics. In recent years the mode-coupling theory (MCT) for the glass transition has been developed [1]. One of its essential predictions is the independence of the glass-transition singularity, as well as of the long-time glassy dynamics, from the microscopic dynamics. This means that the glass-transition singularity and associated slow dynamics are completely determined by the statistics of the system’s orbits in configuration space rather than in phase space [2]. Such dynamics, determined solely by the equilibrium structure, is referred to as the structural relaxation. This MCT prediction agrees with the hypothesis that the potential energy-landscape controls the slow dynamics [3]. Indeed, an interesting characterization of the MCT critical temperature Tc in terms of geometrical properties of the landscape has been proposed [4, 5]. The issue of the independence of the long-time dynamics from underlying microscopic dynamics has been addressed for a binary mixture of Lennard-Jones particles [6]. In Ref. [6], molecular-dynamics (MD) simulations have been performed based on Newtonian as well as stochastic dynamics, and the same long-time relaxation was observed. One of the main points of this Letter is to explore whether such a property holds for molecular systems. This is nontrivial because the dynamics in molecular liquids involves rotational as well as translational motions, and there exist more than two time scales characterizing microscopic dynamics. Since