Electronically from inert gases + halides

We need only know the values of the constants b, a, and d in order to calculate potential energies from Eq. (2-47). The exact values of these constants for alkali metal ions and halide ions are not known. However, the alkali metal ions and the halide ions have inert-gas electronic configurations. For example, if LiF is formulated as an ionic molecule, Li+ is isoelectronic with the inert gas He, and F is isoelectronic with the inert gas Ne. Thus the van der Waals interaction in Li+F may be considered approximately equal to the van... 

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