Computer simulation of the dynamics of aqueous solvation

Abstract

Equilibrium and nonequilibrium molecular dynamics computer simulations have been used to study the time dependence of solvation in water. The systems investigated consisted of monatomic ions immersed in large spherical clusters of ST2 water. Relaxation of the solvation energy following step junction jumps in the solute’s charge, dipole moment, and quadrupole moment have been determined from equilibrium molecular dynamics (MD) simulations under the assumption of a linear solvation response. The relaxation times observed differ substantially depending on the type of multipole jump and the charge/size ratio of the solute. These results could not be quantitatively understood on the basis either of continuum or molecular theories of solvation dynamics currently available. Even the qualitative picture of a distribution of relaxation times which monatonically increases with distance away from the solute is not correct for the systems studied. This lack of agreement is partially explained in terms of the structured environment of the first solvation shell of aqueous solutes. However, translational mechanisms of polarization decay and effects due to the finite distribution of charge within solvent molecules, which should be operative in less structured solvents as well, also contribute to deviations from theoretical predictions. The validity of a linear response approach has been examined for the case of charge jumps using nonequilibrium simulations. The observed dynamics are not generally independent of the size of the charge jump and thus linear response theories are not strictly applicable. In most cases, however, predictions based on a linear response calculation using the equilibrium dynamics of the appropriate reference system still provide a reasonable description of the actual nonequilibrium dynamics.