We investigate the dynamic propensity in a coarse-grained model of a room-temperature ionic liquid via molecular dynamics simulations. Dynamic propensity is defined as the average of squared displacements for each ion during a given time interval over the isoconfigurational ensemble. As the temperature is lowered, distributions of the dynamic propensity develop fat tails at high values, indicating the presence of dynamic heterogeneity in the system. The increase in the heterogeneity for the cation is more evident than that for the anion, and a high propensity exhibits a large variance in the isoconfigurational ensemble, implying that dynamic propensity is related to ions' motions at a large length scale, rather than a direct measure of the individual ion dynamics. In addition, large non-Gaussian parameters observed for small dynamic propensities reveal intermittent dynamical behaviors of ions. In order to reveal the origin of the dynamic heterogeneity in a room-temperature ionic liquid, a possible correlation between the mobility and dynamic propensity is further probed. It is observed that spatial distributions of the dynamic propensity coincide with those of the mobility. The results suggest a possible connection between the structure and heterogeneous dynamics on large length scales.

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Influence of the excitation energy of a probe solute molecule on its solvation dynamics and emission spectrum in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF6 (-)) is studied via molecular dynamics simulations using a coarse-grained model description. By exciting the probe at different energies, each with an extremely narrow distribution, ensuing solvent relaxation and its dynamic variance are monitored using the isoconfigurational ensemble method. Resulting Stokes shift function, S(t), indicates that long-time solvent relaxation becomes slower with the decreasing excitation energy and approaches the equilibrium correlation function, C(t), of solvent fluctuations. This suggests that the system excited at the red-edge of the spectrum observes linear response better than that at the blue-edge. A detailed analysis of nonequilibrium trajectories shows that the effect of initial configurations on variance of relaxation dynamics is mainly confined to short times; it reaches a maximum around 0.1 ≲ t ≲ 1 ps and diminishes as time further increases. The influence of the initial velocity distribution, on the other hand, tends to grow with time and dominates the long-time variations of dynamics. The emission spectrum shows the red-edge effect in accord with previous studies.

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