Systematic Coarse Graining of 4-Cyano-4'-pentylbiphenyl

A coarse-grained model is derived for a liquid-crystal-forming molecule, 4-cyano-4′-pentylbiphenyl (5CB), from a detailed atomistic model using the iterative Boltzmann inversion (IBI) method in the isotropic phase at 315 K and 1 bar. The coarse-grained model consists of five “superatoms” (one for the cyano group, two for the aromatic rings in the biphenyl moiety, and two for the alkyl tail), which are categorized as three types. A modification of IBI, wherein only one of the effective intermolecular potentials (the one corresponding to the superatom pair whose intermolecular correlation function exhibits the highest deviation from the atomistic one) is updated at each iteration, proves to be necessary to achieve convergence. The coarse-grained model, which enables a savings of a factor of 35 in computational cost relative to atomistic simulation, is used to explore ordering into liquid-crystalline phases at lower temperatures. It is found to yield a first-order ordering transition at 288 K with small hysteresis and negligible system size effects. A detailed investigation in terms of various structural and dynamical measurements indicates that the ordered phase is of the smectic type rather than nematic, as observed experimentally. The ordering temperature can be brought close to the experimental value of 308.5 K through the simple rescaling of the intermolecular effective interaction potentials employed in the coarse-grained model. A nematic ordered phase can be obtained from the coarse-grained model by scaling up the head-head and tail-tail effective interaction potentials obtained by IBI.