THE AQUEOUS SOLVATION OF WATER - A COMPARISON OF CONTINUUM METHODS WITH MOLECULAR-DYNAMICS

The calculation of the solvation properties of a single water molecule in liquid water is carried out in two ways. In the first, the water m olecule is placed in a cavity and the solvent is treated as a dielectr ic continuum. This model is analyzed by numerically solving the Poisso n equation using the DelPhi program. The resulting solvation propertie s depend sensitively on the shape and size of the cavity. In the secon d method, the solvent and solute molecules are treated explicitly in m olecular dynamics simulations using Ewald boundary conditions. We find a 2-kcal/mol difference in solvation free energies predicted by these two methods when standard cavity radii are used. In addition, dielect ric continuum theory assumes that the solvent reacts solely by realign ing its electric moments linearly with the strength of the solute's el ectric field; the results of the molecular simulation show important n onlinear effects. Nonlinear solvent effects are generally of two types : dielectric saturation, due to solvent-solute hydrogen bonds, and ele ctrostriction, a decrease in the solute cavity due to an increased ele ctrostatic interaction. We find very good agreement between the two me thods if the radii defining the solute cavity used in the continuum th eory is decreased with the solute charges, indicating that electrostri ction is the primary nonlinear effect and suggesting a procedure for i mprovement of continuum methods. The two methods cannot be made to agr ee when the atomic radii are made charge independent, but charge depen dent cavity radii are shown to greatly improve agreement.