The Aqueous Solvation of Water: A Comparison of Continuum Methods with Molecular Dynamics

摘要

The calculation of the solvation properties of a single water molecule inliquid water is carried out in two ways. In the first, the water molecule isplaced in a cavity and the solvent is treated as a dielectric continuum. Thismodel is analyzed by numerically solving the Poisson equation using the DelPhiprogram. The resulting solvation properties depend sensitively on the shape andsize of the cavity. In the second method, the solvent and solute molecules aretreated explicitly in molecular dynamics simulations using Ewald boundaryconditions. We find a 2 kcal/mole difference in solvation free energiespredicted by these two methods when standard cavity radii are used. Inaddition, dielectric continuum theory assumes that the solvent reacts solely byrealigning its electric moments linearly with the strength of the solute'selectric field; the results of the molecular simulation show importantnon-linear effects. Non-linear solvent effects are generally of two types:dielectric saturation, due to solvent-solute hydrogen bonds, andelectrostriction, a decrease in the solute cavity due to an increasedelectrostatic interaction. We find very good agreement between the two methodsif the radii defining the solute cavity used in the continuum theory isdecreased with the solute charges,