  The molecular dynamics (MD) simulation study of solvation structure and free
energetics in 1-ethyl-3-methylimidazolium chloride and
1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the
preceding article [Y. Shim, M. Y. Choi and H. J. Kim, cond-mat/0501305] is
extended to investigate dynamic properties of these liquids. Solvent
fluctuation dynamics near equilibrium are studied via MD and associated
time-depedent friction is analyzed via the generalized Langevin equation.
Nonequilibrium solvent relaxation following an instantaneous change in the
solute charge distribution and accompanying solvent structure reorganization
are also investigated. Both equilibrium and nonequilibrium solvation dynamics
are characterized by at least two vastly different time scales--a subpicosecond
inertial regime followed by a slow diffusive regime. Solvent regions
contributing to the subpicosecond nonequilibrium relaxation are found to vary
significantly with initial solvation configurations, especially near the
solute. If the solvent density near the solute is sufficiently high at the
outset of the relaxation, subpicosecond dynamics are mainly governed by the
motions of a few ions close to the solute. By contrast, in the case of a low
local density, solvent ions located not only close to but also relatively far
from the solute participate in the subpicosecond relaxation. Despite this
difference, linear response holds reasonably well in both ionic liquids.
