Utopia dielectrica

The dielectric constant of a material is a macroscopic property that measures the reduction of the electrostatic forces between charged plates separated by the material, compared to a vacuum as intermediate material. It is next encountered as a scaling parameter in Coulomb's law for interacting charges, not only in the force, but also in the energy. In deriving the theory for dielectrics, the macroscopic nature is essential: Only then is the basic assumption that the dielectric material is homogeneous and isotropic a valid one. The appearance of the dielectric constant as a simple scaling factor in Coulomb's law has tempted many computational chemists to forget about the macroscopic nature of the dielectric and to apply the screened Coulomb's law between charges, supposedly in a low;dielectric medium such as proteins, in microscopic force fields. Optimization of force fields even led to distance-dependent ''dielectric constants.'' Another use of the dielectric constant appears in the dielectric continuum reaction field approaches for the computations of solvation energies and solvent effects. The solute is embedded in a cavity surrounded by the dielectric. Specific interactions between solvent molecules and solute are thus neglected. The cavity size and dielectric constants of interior and exterior are optimized for the model. The aim of this article is to show, by means of calculations on interacting point charges embedded in cavities surrounded by dielectrics and microscopic models of ''low-dielectric'' materials by explicit polarizabilities, that as far as the dielectric ''constant'' is concerned anything can happen, depending on the nature of the charges, the distance to the cavity boundary, the spatial arrangement of charges, and polarizabilities. Thus, a warning is issued to injudicious use of dielectric models in microscopic calculations. (C) 1995 John Wiley & Sons, Inc.