Combination rules and correlations in repulsive potential parameters for alkali halide diatomics

The single-ion contributions, ${\rho }_i$, to alkali ($M$) halide ($X$) repulsive interactions of the form $A_{\mathrm{MX}}\mathrm{}\left(R\right)\mathrm{}{e}^{\frac{-R}{{\rho }_{\mathrm{MX}}}}$, with ${\rho }_{\mathrm{MX}}={\rho }_M+{\rho }_X$, are considered for two diatomic models. The ${\rho }_i$ values are shown to correlate linearly with the position of the outermost peak of the electron density distribution of the ion. A similar trend is observed in the single-ion contributions to the range parameter for the square of the $M-X$ overlap. Diatomic single-ion parameters are shown to differ for the two models although the combination rules are equally well obeyed. Single-ion contributions to the pre-exponential factors, "soft-sphere" radii, and repulsive interaction potentials for $M-M$ and $X-X$ are briefly discussed.