Symmetries of the electrodynamic interactions between chiral molecules

Abstract

A quantum electrodynamics calculation of the dynamic microscopic interactions between molecules beyond the electric dipole approximation is presented. The symmetry of the resulting terms is examined with respect to rotations, parity and index permutations. Their relative importance is also considered in the short-distance limit. A dominant term which exists only in chiral molecules and which involves the coupling of the anisotropic parts of the molecular optical activities is evidenced. Associated with the coupling of the anisotropic parts of the molecular polarisabilities which is usually invoked to account for the isotropic–nematic phase transition of liquid crystals, that term accounts for the possibility of the helical order which is observed in chiral nematics (cholesterics). Introducing in addition both the short-range correlations due to the exclusion principle driven repulsive forces and the thermally-induced effect of the static multipole interactions can explain the temperature dependence of the helical pitch. No assumption will be made on the shape of the molecules.