Conformation energy surface for liquid crystal molecules from first principles: Application to 5CB

We have performed first principles molecular dynamics calculations to investigate the structure and conformation of the liquid crystal molecule 5CB which contains a pentane chain joined to two phenyl rings terminated by a cyanide group. We describe the electronic structure by using density functional theory within the generalised gradient approximation for electron exchange and correlation and expand the electronic wavefunctions in a plane wave basis set. Structural optimisation of atomic positions is performed to obtain the relaxed molecular geometry. The relative angle between the two phenyl rings and the angle of the pentane tail is allowed to vary. For each possible pair of angles we calculate the total energy of the structure from which we obtain the energy surface for these conformations. We find the optimum conformation of the molecule is non-planar with a relative angle between the phenyl rings of 31 degrees, in excellent agreement with recent NMR data. The angle of the pentane tail has a small, but significant effect on the energetics of the molecule. We fit accurate intra-molecular potentials for biphenyl and 0CB which can be used in large empirical simulations of liquid crystals. We compare the results with similar calculations performed on liquid crystal fragments.