The shells theory proposed by Pliego is an example of a hybrid implicit/explicit solvation method . In this solvation treatment, the solvent shell closest to the solute (S 1 ), representing solute-solvent interaction, is treated either fully quantum mechanically, as in Figure , or with molecular dynamics based on classical force fields, as in Figure . The remaining solvent, S 2 , is treated with continuum solvation. When the number of solvent molecules in S 1 becomes infinite, this theory converges to the full discrete solvent representation. This approach is also known as the cluster-continuum model, mixed discrete-continuum model, or quasichemical theory .