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,
SCRF)[7].
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 [6].
References: