Ionic contribution to van der Waals interaction of polar compounds

Ionic contributions to the van der Waals interactions of typical polar compounds have been investigated based on density-functional-theory-calculated dielectric functions. Derived from the Lifshitz theory, the contribution of ionic polarizabilities to the dispersion energy between parallel half-spaces can be up to 3.4%. Using the Clausius-Mossotti relationship with a Lorentz factor of 4p/3, it is found that the ionic effect on C6 is consistent with that on energy. In parallel slabs separated distantly, the ionic polarizability plays a substantially enhanced role, contributing up to 25% of the energy. The significant ionic effect in the latter case is due to a reduction in the screening of low-frequency terms, and a zero Lorentz factor should be assumed to predict a consistent ionic effect on C6.