The effect of a solvent can be incorporated in quantum-chemical
calculations most easily by considering it as a continuous
dielectric medium, characterized by a dielectric constant.
The electric field caused by the molecule induces a
polarization of the medium, which in turn acts on the
electrons in the molecule (Self-Consistent Reaction Field,
The model thus contains the quantum-mechanical description of the molecule and a classical medium. The problem is to choose where to locate the boundary between quantum system and medium. In the Gaussian programs a simple approximation is used in which the volume of the solute is used to compute the radius of a cavity which forms the hypothetical surface of the molecule. Spartan offers solvation models for the semi-empirical Hamiltonians.
This method, in which the molecular surface is constructed from atomic (Born) radii, requires parameters for each atom [6, 20]. In most cases, solvation hardly affects the structure of a molecule (relative to the gas phase), but in cases of polar molecules, zwitterions or ions, the relative energies can be changed dramatically .