Fig. 2.

Schematic view of the interaction between the ions. Each atom iβ is approximated by a sphere with radius a. The dielectric constant inside the sphere ∈^sp is one, outside the sphere ∈^con equals ∈_∞. In other words, the spherical atom with its partial charge $q_{i\beta }^{\text{perm}}$ is immersed in a dielectric continuum with ∈_∞. For further details the reader is referred to the Appendix.