CONFORMATION-DEPENDENT DIPOLES OF LIQUID-CRYSTAL MOLECULES AND FRAGMENTS FROM FIRST PRINCIPLES

We determine accurate molecular dipole moments for mesogenic fragments and liquid crystal molecules from quantum mechanical computer simulat ions adapted from large-scale electronic structure calculations on per iodic solids. We employ density functional theory and use ab initio ps eudopotentials for the interaction between valence electrons and ions and the generalized gradient approximation to account for the many-bod y effects of exchange and correlation. Periodic boundary conditions ar e enforced so that the molecular electronic wave function can be expan ded in terms of a plane wave basis set. We test our method on several small molecules and then apply it to determine the direction, position and dipole moment magnitude for 4-4' pentyl-cyanobiphenyl (5CB) (and related fragments), phenyl benzoate, and 2-2'difluorobiphenyl. For the latter compound, we parametrize a torsional potential for rotation ab out the dihedral bond. We perform full structural optimization, and fi nd that the torsional barrier heights for fully relaxed molecular stru ctures are substantially reduced relative to nonoptimized geometries. We then demonstrate the influence of conformation and temperature on t he molecular dipole moment. We also find that simple vector addition o f dipole moments of fragments provides a good estimate of the total di pole moment of the complete molecule. We compare our results to experi ment and to conventional quantum chemistry methods where data is avail able.