Validating an optimized GAFF force field for liquid crystals: T_NI predictions for bent-core mesogens and the first atomistic predictions of a dark conglomerate phase

Abstract

The GAFF-LCFF force field [N. J. Boyd et al., Phys. Chem. Chem. Phys., 2015, 17, 24851] is tested and further improved for use in the simulation of bent-core liquid crystal mesogens. Atomistic simulations are carried out on four systems of bent-core nematogens based on a central bis-(phenyl)oxadiazole (ODBP) motif, providing excellent agreement with experimental, T, transition temperatures. Simulations of one bent-core system (C5-Ph-ODBP-Ph-OC12) indicate the presence of a dark conglomerate (DC) phase, with the prediction of a highly unusual nematic to DC phase transition.

Emergent chirality in achiral liquid crystals: insights from molecular simulation models of the behaviour of bent-core mesogens

“…Whilst φ 2 prefers angles of AE 45 and AE 135 corresponding to a nonplanar arrangement. This nonplanar preference coupled with a preferred nonplanar arrangement of linking ester groups [25,28] gives rise to twisted conformations for many bent-core molecules, which, following Kawauchi et al [28], we suggest is important in contributing to the nature of the neighbour-neighbour interactions exhibited by these molecules.…”

“…Atomistic MD simulations were carried out using the GROMACS 4.5.5 package [23]. The calculations use our own modified version of the GAFF force field (GAFF-LCFF), which has been developed in two recent publications [24,25], including for bent-core mesogens [25]. Table 1.…”

“…The Antechamber software from AmberTools 1.4 was used to generate GAFF topologies, with the point charges derived from the AM1-BCC method. GAFF-LCFF force field parameters from [24,25] were added in this stage. The GAFF topologies and coordinate files were converted into the GROMACS format using the ACPYPE script [26].…”

“…The original GAFF force field parameters do not accurately represent the two dihedral angles φ 1 and φ 2 within the benzalaniline (N-benzylideneaniline) fragment, as shown in Figure 1. Moreover, GAFF-LCFF [24,25] has not yet been extended to include these. To rectify this, we calculated torsional parameters from electronic structure theory calculations and fitted these to atomistic potentials.…”

Assessing the transferability of common top-down and bottom-up coarse-grained molecular models for molecular mixtures

“…The modified force field, GAFF-LCFF, was parametrised by Boyd and Wilson to accurately capture the experimental densities and heats of vaporization of a range of molecules, including medium chain alkanes. 41,42 All simulations were performed using the GRO-MACS 4.6 package. 43 We used a leap-frog algorithm, with a time step of 2 fs.…”