  Recent developments in molecular theories and simulation of ions and polar
molecules in water are reviewed. The hydration of imidazole and imidazolium
solutes 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 get correct hydration
free energies in the thermodynamic limit from molecular calculations; and that
molecular simulations with Ewald interactions and periodic boundary conditions
can also be more efficient than many common alternatives. The Ewald treatment
permits a conclusive extrapolation to infinite system size. The picture that
emerges from testing of simple models is that the most prominent failings of
the simplest theories are associated with solvent proton conformations that
lead to non-gaussian fluctuations of electrostatic potentials. Thus, the most
favorable cases for the second-order perturbation theories are monoatomic
positive ions. For polar and anionic solutes, continuum or gaussian theories
are less accurate. The appreciation of the specific deficiencies of those
simple models have led to new concepts, multistate gaussian and quasi-chemical
theories, that address the cases for which the simpler theories fail. It is
argued that, relative to direct dielectric continuum treatments, the
quasi-chemical theories provide a better theoretical organization for the
computational study of the electronic structure of solution species.
