Soft‐Sphere Model for Closed‐Shell Atoms and Ions

Using recent spectroscopic data for the alkali halide monomers, it is shown that approximate additivity rules of the form Rij = Ri + Rj and ρij = ρi + ρj hold for the radii and hardness parameters in the Born–Mayer repulsive potential, Uij(R) = fρijexp[(Rij − R) / ρij].