The dynamic terms in induced circular dichroism

Circular dichroism can be induced in an achiral molecule both by the static and time-dependent fields of neighbouring chiral molecules. Here the time-dependent pairwise interactions are calculated by quantum electro-dynamics for all separations of the interacting pair. It is shown how, by two alternative canonical transformations of the Hamiltonian, the full results can be recovered much more simply, with advantages also in physical insight. For molecule pairs in fixed relative orientations the leading term varies with distance as $R^{-3}$. Various orientational averages are also calculated, applicable to pairs in the gas phase coupled weakly by intermolecular forces. For coupling either through permanent electric dipoles, or by the dispersion force, the leading term is in $R^{-9}$; in the first the dependence on temperature is on $T^{-2}$, in the second on $T^{-1}$.