  We calculate the effective force between two oppositely charged and similarly
charged ions fixed in water as a function of separation distance R. At short
separations, R less than 1 nm, the effective force is vastly different from the
$1/{\epsilon}_{s}R^2$ dependence advocated by the screened Coulomb force law
(SCFL); ${\epsilon}_{s}$ being the static dielectric constant of the medium. We
also find an interesting asymmetry in the force between +ve and -ve ions. This
breakdown of SCFL is shown to be due to the persistent interference between the
polarizations created by the two charges, in a manner similar to the
vortex-antivortex pair formation in XY model Hamiltonian. The distance
dependence of dielectric constants, ${\epsilon}_{s}$(R) extracted from our
simulation exhibit interesting features and can be used in future modelling.
