Comparison of the third-order dispersion terms of interaction between chiral molecules

Using a quantum electrodynamic approach, the coupling between chiral molecules derived from the exchange of three virtual photons of the electric dipole type is investigated. The resulting interaction potential, which involves the first nonlinear polarizability of the molecules, is compared with a previously derived interaction potential involving the optical activity of the molecules; the latter is accounted for by the exchange of two virtual photons, one being electric dipolar and the other being electric quadrupolar or magnetic dipolar. The symmetry properties of the new term is considered and its quasi-static limit is derived. It appears that the two interaction terms, which both are third-order dispersion processes, might have the same order of magnitude; thus none can a priori be neglected and both might equally contribute to the possibility of a helical ordering.