Computer Simulations of a Twist Bend Nematic (N_TB): A Coarse-Grained Simulation of the Phase Behaviour of the Liquid Crystal Dimer CB7CB

Abstract

In recent years, a number of achiral liquid crystal dimer molecules have been shown to exhibit nematic–nematic phase transitions. The lower temperature phase has been identified as the NTB phase, which demonstrates emergent chirality in the spontaneous formation of a heliconical structure. Recent fully atomistic simulations of the molecule CB7CB (1,7-bis-4-(4′-cyanobiphenyl) heptane), a dimer with an odd number of carbon spacers between the mesogenic parts of the molecule, have captured the NTB–N–I phase sequence, providing a picture of the order at a molecular level. In this paper, we use atomistic simulations of CB7CB to develop a coarse-grained model using systematic coarse graining in the NTB phase. We use both force matching (in the form of the MS-CG method) and iterative Boltzmann inversion (IBI) methodologies. Both techniques capture the heliconical order within the NTB phase. Moreover, the model developed via force matching is shown to provide an excellent representation of the atomistic simulation reference model and, remarkably, demonstrates good transferability across temperatures, allowing the NTB–N and N–I phase transitions to be simulated. We also compare results with those of a Martini 3-based coarse-grained model.

Keywords

• liquid crystals
• twist-bend nematic
• coarse-graining
• dimer
• molecular dynamics