A MOLECULAR CONTINUUM THERMODYNAMIC MODEL OF HYDRATION/

We propose an integrated molecular/continuum thermodynamic model for c alculating electrostatic contributions to the free energy of hydration that is based on a correlation function expansion of the free energy. The model uses explicit water simulation results for simple monatomic and diatomic solutes to calculate lower order correlations in this ex pansion and systematically corrects for higher order correlations usin g the continuum description of hydration. The model is applied to calc ulating free energies of charging the tetramethylammonium (TMA) ion an d idealized TMA-like solutes in water. We find that the effects of wat er structure on ion hydration are embodied in the lower order correlat ions and, thus, are accounted for by the explicit water simulation res ults. In addition, continuum corrections for higher order correlations are found to be independent of the Born radii chosen for oppositely c harged ions. Thus, the model reduces the reliance of the continuum des cription on parametrization of the Born radii.