MOLECULAR THEORIES AND SIMULATION OF IONS AND POLAR-MOLECULES IN WATER

Recent developments in molecular theories and simulation of ions and p olar molecules in water are reviewed. The hydration of imidazole and i midazolium is used to exemplify the theoretical issues. The treatment of long-ranged electrostatic interactions in simulations is discussed extensively. It is argued that the Ewald approach is an easy way to ge t correct hydration free energies corresponding to thermodynamic limit from molecular calculations. Molecular simulations with Ewald interac tions and periodic boundary conditions can also be more efficient than many common alternatives. The Ewald treatment permits a conclusive ex trapolation to infinite system size. Accurate results for well-defined models have permitted careful testing of simple theories of electrost atic hydration free energies, such as dielectric continuum models. The picture that emerges from such testing is that the most prominent fai lings of the simplest theories are associated with solvent proton conf ormations that lead to non-Gaussian fluctuations of electrostatic pote ntials. Thus, the most favorable cases for second-order perturbation t heories are monoatomic positive ions. For polar and anionic solutes, c ontinuum or Gaussian theories are less accurate. The appreciation of t he specific deficiencies of those simple models have led to new concep ts, multistate Gaussian and quasi-chemical theories, which address the cases for which the simpler theories fail. It is argued that, relativ e to direct dielectric continuum treatments, the quasi-chemical theori es provide a better theoretical organization for the computational stu dy of the electronic structure of solution species.