Numerical simulations of solvation in simple polar solvents Results and discussion

5 NUMERICAL SIMULATIONS OF SOLVATION IN SIMPLE POLAR SOLVENTS RESULTS AND DISCUSSION 

These results show a large degree of similarity between the linear response (equilibrium) and nonequilibrium results. Both consist of an initial fast relaxation mode that, at closer inspection is found to be represented well by a Gaussian, exp[-(t/x) ], followed by a relatively slow residual relaxation. The initial fast part is more pronounced in C(t). The latter is also characterized by stronger oscillations in the residual part of the relaxation. The fact that the linear response results obtained for equilibrium simulations with an uncharged solute and with a solute of charge q are very similar give further evidence to the approximate validity of linear response theory for this systems. 

The namre of this fast relaxation component of the solvation dynamics has been discussed extensively in the past decade. Carter and Hynes were the first to suggest that this initial relaxation is associated with inertial, as opposed to diffusive, solvent motion. In this mode of motion solvent molecules move under the suddenly created force-field without their mutual interactions and consequent energy exchange having a substantial influence on this fast time scale. Simulations and analytical calculations have confirmed this assertion for simple polar fluids. 

Next we examine the relative contributions of different solvation shells to the solvation process. This issue is important for elucidating the solvation mechanism, and has been imder discussion since an early remark made by Onsager that the shorter time scales are associated mostly with solvent layers further away from the solute, and that the longer Tq times are associated with the individual response of sol- 

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