Phase Transitions and Hysteresis for a Simple Model Liquid Crystal by Replica-Exchange Monte Carlo Simulations

Abstract

In this work, the advantages of applying the temperature and pressure replica-exchange method to investigate the phase transitions and the hysteresis for liquid-crystal fluids were demonstrated. In applying this method to the commonly used Hess–Su liquid-crystal model, heat capacity peaks and points of phase co-existence were observed. The absence of a smectic phase at higher densities and a narrow range of the nematic phase were reported. The identity of the crystalline phase of this system was found to a hexagonal close-packed solid. Since the nematic-solid phase transition is strongly first order, care must be taken when using this model not to inadvertently simulate meta-stable nematic states at higher densities. In further analysis, the Weighted Histogram Analysis Method was applied to verify the precise locations of the phase transition points.

Hysteresis Elimination for an Anisotropic Liquid-Crystal Model via Molecule Design and Replica-Exchange Optimization

“…In our work, in 2021, 14 we performed PT-REM simulations for the same Hess−Su liquid-crystal model used here. No optimization of replica spacing was performed, and replicas with temperatures higher and lower than the baseline were prepared for both upward and downward branch systems.…”

“…smectic phase exhibits dynamic behavior indicative of liquid flow and yet is highly ordered. Thus, it is considered a challenge to reproduce these opposing properties in molecular simulations accurately, and we encountered these difficulties when simulating both soft rods 14 and hard rods. 15 Many molecular representations of liquid crystals have been proposed and simulated, ranging from all-atom models 16,17 and united-atom models 18,19 to coarse-grained models.…”

“…This technique increases the likelihood of phase transitions occurring compared to conventional methods and has been successfully applied to liquid crystals. − Liquid crystals are typified as materials that may form intermediate phases between amorphous and crystalline. For example, among these intermediate phases, the smectic phase exhibits dynamic behavior indicative of liquid flow and yet is highly ordered. Thus, it is considered a challenge to reproduce these opposing properties in molecular simulations accurately, and we encountered these difficulties when simulating both soft rods and hard rods . Many molecular representations of liquid crystals have been proposed and simulated, ranging from all-atom models , and united-atom models , to coarse-grained models. , Complications arising from anisotropy have led to challenges concerning the overcoming of hysteresis in phase transitions or molecular conformations.…”

“…Since the PT-REM can expand and reduce the system volume, something not possible with the canonical REM, this allows a broader energy landscape to be explored and is potentially more helpful for pressure-sensitive systems. In our previous study, we applied pressure and temperature replica exchange to the Hess–Su coarse-grained liquid-crystal model. Exploring a wide range of energy spaces was relatively straightforward compared with conventional Monte Carlo simulations.…”

Simulating the nematic-isotropic phase transition of liquid crystal model via generalized replica-exchange method

“…It should be noted that various enhanced sampling methods beyond the original tREM have been developed. 15 Berardi et al 16 and Kowaguchi et al 17 attempted to improve the sampling efficiency near the NI phase transition with the Gay-Berne model and anisotropic Lennard-Jones fluids using the multidimensional REM 18 (also known as Hamiltonian REM 19 ) and the isobaric-isothermal REM, 20 respectively.…”