Condensed-phase Effects on Structure and Reactivity

The gas phase is delightful in its simplicity. At low to moderate pressures, molecules may be treated as isolated, non-interacting species, and this facilitates theoretical modeling enormously, insofar as the system of interest is entirely defined by die molecule itself. Were theory to restrict itself to the gas phase, however, it would be inapplicable to vast tracts of chemistry, to include essentially all of biochemistry. 

Essentials of Computational Chemistry, 2nd Edition Christopher J. Cramer 

The assumption underlying continuum solvation models, which are the subject of this chapter, is that one may remove the huge number of individual solvent molecules from the model, as long as one modifies the space those molecules used to occupy so that, modeled as a continuous medium, it has properties consistent witli those of the solvent itself. To determine how to define such a medium, one must consider the solvation process itself. 

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