Quadrupole Contributions to London's Dispersion Forces

The contribution of terms in R—8 and R—10 to the dispersion forces is expressed by a simple, approximate formula (Eq. (7)) involving only measurable quantities (polarizability, absorption frequency, oscillator strength). The formula is applicable when the dispersion curve of the substance can be represented with the use of a single resonant frequency. Numerical values of the terms in question are calculated for a number of molecules (Table II). Except in the case of the alkalies, where the convergence of the sequence in inverse powers of R fails at distances around 6A, the R—10 term is generally negligible, while the R—8 term contributes appreciably in the region of the van der Waals minimum.