Optimization of the GAFF force field to describe liquid crystal molecules: the path to a dramatic improvement in transition temperature predictions

Abstract

The physical properties and phase transitions of thermotropic liquid crystals are highly sensitive to small changes in chemical structure. However, these changes are challenging to model, as both the phase diagram and mesophase properties obtained from fully atomistic simulations are strongly dependent on the force field model employed, and the current generation of chemical force fields has not proved accurate enough to provide reliable predictions of transition temperatures for many liquid crystals. This paper presents a strategy for improving the nematic clearing point, TNI, in atomistic simulations, by systematic optimization of the General Amber Force Field (GAFF) for key mesogenic fragments. We show that with careful optimization of the parameters describing a series of liquid crystal fragment molecules, it is possible to transfer these parameters to larger liquid crystal molecules and make accurate predictions for nematic mesophase formation. This new force field, GAFF-LCFF, is used to predict the nematic-isotropic clearing point to within 5 °C for the nematogen 1,3-benzenedicarboxylic acid,1,3-bis(4-butylphenyl)ester, an improvement of 60 °C over the standard GAFF force field.

Dyes as guests in ordered systems: current understanding and future directions

“…[84,85] There are now multiple reports of simulations of guest molecules within liquid crystal hosts, providing favourable comparisons with results of electron paramagnetic resonance [86][87][88] and nuclear magnetic resonance [89,90] experiments, as well as providing assessments of the validity of mean field models of molecular alignment. [91] The increase in computational resources has also made electronic structure calculations more accessible with time.…”

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

“…0.64 for E7 alone [29,64], but they provide a significant improvement on that of ca.0.88 we obtained when using the OPLS (Optimised Potential for Liquid Simulations) force field in earlier studies [29]. This improvement may be primarily attributed to the weaker Lennard-Jones interactions of the aromatic carbon atoms in the adapted GAFF force field used here [51]. The molecular order parameters, S θ (MOI), of the minimum MOI axes of the dye molecules in the guest-host simulations, determined against the host director and calculated according to the first term of Equation (4), are shown in the left plots in Figure 9.…”

“…Simulations were run from pseudo-nematic starting geometries with five dye molecules in 400 host molecules, as described previously [29], and trajectory frames were recorded every 25 ps. Order parameters of the host molecules were defined as twice the negative of the middle eigenvalue of the Q-tensor [51], and plots of order parameter against time are shown as 20-point averages for clarity. Standard errors of the order parameters determined from the MD simulations were calculated using the method described by Chodera [59], and gave standard errors between 0.004 and 0.045, dependent on the data set and the particular order parameter calculated; these order parameters are given consistently to three decimal places, and the standard errors are listed in full in the supplementary information.…”

“…Structure optimisations and TD-DFT calculations were carried out at the PBE0/6-311G(d,p) level [40,41], which has been shown generally to be successful for the prediction of visible absorption spectra of organic dyes [42]. Fully atomistic MD simulations were carried out using GROMACS 5.1.2 [43][44][45][46][47][48][49], and the General Amber Force Field (GAFF) [50], with adjustments reported for the simulation of liquid crystal phases [51]. Atomic charges were determined using the RESP method for the optimised structures [52].…”

Simulation insights into the role of antiparallel molecular association in the formation of smectic A phases

“…[16][17][18] For nematics, it is now possible to generate very good predictions for transition temperatures from atomistic simulations of liquid crystal phases. [19][20][21] However, for more complex mesophases, coarse-grained models are extremely useful; particularly because the system sizes and time scales required are usually too demanding for atomistic level models. While excluded volume factors play a major role in driving mesophase formation, specific pair-wise interactions have long been known to provide an important contribution to the formation of some liquid crystal phases.…”