A universal scaling scheme is developed for closed-shell interactions. The exchange energies (total energies minus the Coulombic energies) are found to scale with two parameters to universal interaction curves for noble gas–noble gas, alkali ion–noble gas, and halogen ion–noble gas interactions. The interaction potentials constructed from the universal interaction curves agree well with experimentally determined potentials, and also successfully reproduce measured ion mobilities and diffusion coefficients. The universal interactions can be viewed not just as a correlation scheme, but also as operating to extend the range of the potentials for a number of ion–atom systems to both larger and smaller distances than are presently probed by direct measurements. They also provide the basis for predictions of potentials for systems lacking experimental measurements. In the case of the noble gases, they reduce by two the number of parameters required for the formulation of an accurate extended principle of corresponding states.

Combination rules for dispersion coefficients are greatly extended by way of a parameter corresponding to an effective number of equivalent electrons, the same parameter N that appears in the Slater–Kirkwood formula for the dipole–dipole coefficient. The same N can be used for all members of an isoelectronic sequence, and new formulas are given for higher-order two-body dispersion coefficients and three-body nonadditive coefficients, in which the same N can also be used. The only additional input data needed are static multipole polarizabilities. A theoretical justification is given using screening-constant wave functions. Extensive empirical testing suggests that the results are usually accurate within about 5% for dipole coefficients and about 5%–10% for higher coefficients. The results apply only to S-state atoms and ions, but should be capable of extension to other systems.

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