van der Waals Interactions in Cholesteric Liquid Crystals
Microscopic calculations of the pitch of cholesteric liquid crystals are based on a few types of interactions between molecules: (1) short-range repulsive, (2) direct Coulomb, and (3) long-range van der Waals interactions. Recently, it was shown that first two types cannot be treated in the frame of mean-field approximation. Here we show that, contrary to common belief, an accurate evaluation of the intermolecular dispersion forces contributing to chiral ordering requires consideration of biaxial correlations between molecules which are neglected in the mean-field approximation. We found that in the presence of biaxial correlations chiral interactions depend very weakly on the anisotropy of the local (i.e., atomic) polarizability. Instead, the chiral interaction between two molecules is dominated by the character of biaxial correlations, the isotropic part of local polarizability of one molecule, and a chiral parameter of the other molecule, which characterizes the chiral molecular geometry and is similar to that found previously for steric interactions.