Conformation-dependent dipoles of liquid crystal molecules and fragments from first principles

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We determine accurate molecular dipole moments for mesogenic fragments and liquid crystal molecules from quantum mechanical computer simulations adapted from large-scale electronic structure calculations on periodic solids. We employ density functional theory and use ab initio pseudopotentials for the interaction between valence electrons and ions and the generalized gradient approximation to account for the many-body effects of exchange and correlation. Periodic boundary conditions are enforced so that the molecular electronic wave function can be expanded 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 about the dihedral bond. We perform full structural optimization, and find that the torsional barrier heights for fully relaxed molecular structures are substantially reduced relative to nonoptimized geometries. We then demonstrate the influence of conformation and temperature on the molecular dipole moment. We also find that simple vector addition of dipole moments of fragments provides a good estimate of the total dipole moment of the complete molecule. We compare our results to experiment and to conventional quantum chemistry methods where data is available.