The next challenge to computational calculation of solvent effects is the representation of the solvent and how its presence affects the energy of chemical species. Computational methods for estimating solvation energies are reviewed in and are depicted in Figure . They generally fall into two categories: those that represent solute and solvent molecules explicitly (Figures 3a-c), and those that represent only solute molecules explicitly and the solvent molecules somewhere in between explicit and implicit (Figures ). Explicit treatment is done either quantum mechanically (QM) (Figure ), with molecular mechanics (MM) (Figure ), or some combination of both, as in QM/MM (Figure ). Continuum solvation models represent solvation as a solute placed inside a cavity within an implicit solvent, which is modeled as a continuum with a constant property such as conductivity or dielectric constant (Figure ). The solute cavity can be shaped like a sphere or ellipsoid, or as in more modern methods, based upon a superposition of atom-centered spheres . However, representing a solvent this way does not account for local solutesolvent interactions, and the assumption that the dielectric constant near the solute surface is equal to the bulk dielectric constant is inaccurate.