Modeling a Liquid Crystal Dynamics by Atomistic Simulation with an Ab Initio Derived Force Field

Predicting Spontaneous Orientational Self-Assembly: In Silico Design of Materials with Quantum Mechanically Derived Force Fields

De novo design of self-assembled materials hinges upon our ability to relate macroscopic properties to individual building blocks, thus characterizing in such supramolecular architectures a wide range of observables at varied time/length scales. This work demonstrates that quantum mechanical derived force fields (QMD-FFs) do satisfy this requisite and, most importantly, do so in a predictive manner. To this end, a specific FF, built solely based on the knowledge of the target molecular structure, is employed to reproduce the spontaneous transition to an ordered liquid crystal phase. The simulations deliver a multiscale portrait of such self-assembly processes, where conformational changes within the individual building blocks are intertwined with a 200 ns ensemble reorganization. The extensive characterization provided not only is in quantitative agreement with the experiment but also connects the time/length scales at which it was performed. Realizing QMD-FF predictive power and unmatched accuracy stands as an important leap forward for the bottom-up design of advanced supramolecular materials.

Automated Parameterization of Quantum Mechanically Derived Force Fields for Soft Materials and Complex Fluids: Development and Validation

The reliability of molecular dynamics (MD) simulations in predicting macroscopic properties of complex fluids and soft materials, such as liquid crystals, colloidal suspensions, or polymers, relies on the accuracy of the adopted force field (FF). We present an automated protocol to derive specific and accurate FFs, fully based on ab initio quantum mechanical (QM) data. The integration of the Joyce and Picky procedures, recently proposed by our group to provide an accurate description of simple liquids, is here extended to larger molecules, capable of exhibiting more complex fluid phases. While the standard Joyce protocol is employed to parameterize the intramolecular FF term, a new automated procedure is here proposed to handle the computational cost of the QM calculations required for the parameterization of the intermolecular FF term. The latter is thus obtained by integrating the old Picky procedure with a fragmentation reconstruction method (FRM) that allows for a reliable, yet computationally feasible sampling of the intermolecular energy surface at the QM level. The whole FF parameterization protocol is tested on a benchmark liquid crystal, and the performances of the resulting quantum mechanically derived (QMD) FF were compared with those delivered by a general-purpose, transferable one, and by the third, "hybrid" FF, where only the bonded terms were refined against QM data. Lengthy atomistic MD simulations are carried out with each FF on extended 5CB systems in both isotropic and nematic phases, eventually validating the proposed protocol by comparing the resulting macroscopic properties with other computational models and with experiments. The QMD-FF yields the best performances, reproducing both phases in the correct range of temperatures and well describing their structure, dynamics, and thermodynamic properties, thus providing a clear protocol that may be explored to predict such properties on other complex fluids or soft materials.

Comparison of the accuracy of periodic reaction field methods in molecular dynamics simulations of a model liquid crystal system

A periodic reaction field (PRF) method is a technique to estimate long-range interactions. The method has the potential to effectively reduce the computational cost while maintaining adequate accuracy. We performed molecular dynamics (MD) simulations of a model liquid-crystal system to assess the accuracy of some variations of the PRF method in low-charge-density systems. All the methods had adequate accuracy compared with the results of the particle mesh Ewald (PME) method, except for a few simulation conditions. Furthermore, in all of the simulation conditions, one of the PRF methods had the same accuracy as the PME method.

Parametrization and Validation of Intramolecular Force Fields Derived from DFT Calculations

The energy and its first and second geometrical derivatives obtained by DFT calculations for a number of conformations of a single molecule are used to parametrize intramolecular force fields, suitable for computer simulations. A systematic procedure is proposed to adequately treat either fully atomistic or more simplified force fields, as within the united atom approach or other coarse grained models. The proposed method is tested and validated by performing molecular dynamics simulations on several different molecules, comparing the results with literature force fields and relevant experimental data. Particular emphasis is given to the united atom approach for flexible molecules characterized by "soft" torsional potentials which are known to retain a high degree of chemical specificity.

Non-Newtonian rheological properties of shearing nematic liquid crystal model systems based on the Gay–Berne potential

The logarithm of the viscosity of a nematic liquid crystal is a linear function of the square root of the shear rate in the non-Newtonian regime.

Liquid Crystal Properties of the n-Alkyl-cyanobiphenyl Series from Atomistic Simulations with Ab Initio Derived Force Fields

Lengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different temperatures for the series of the 4-n-alkyl-4'-cyanobiphenyls (nCB) with n = 6, 7, and 8. The accurate atomistic force field (Bizzarri, M.; Cacelli, I.; Prampolini, G; Tani, A. J. Phys. Chem. A 2004, 108, 10336), successfully employed to reproduce thermodynamic and transport properties of the 5CB molecule, has here been extended to higher homologues. Nematic and isotropic phases were found for all members of the series, and also, a smectic phase was (tentatively) identified for 8CB at 1 atm and 300 K. Transition temperatures reproduce the experimental values within +/-10 K. Also, structural properties as second and fourth rank orientational order parameters are in good agreement with the corresponding experimental quantities. This means that the well-known odd-even effect, observed for many properties along the nCB series, is well reproduced, despite the narrow range of oscillations, e.g., in clearing temperatures. A detailed analysis of the correlation between molecular properties and odd-even effects is presented.

Molecular orientational and dipolar correlation in the liquid crystal mixture E7: a molecular dynamics simulation study at a fully atomistic level

Molecular dynamics simulations are reported for the four component nematic liquid crystal mixture E7, which is used commercially. We are able to show the growth of a nematic phase directly from an isotropic liquid over a 100 ns period for an all-atom model, and study orientational and dipole order within the nematic phase. The simulations show that the cyanoterphenyl component of the mixture, 5CT, is more ordered than the three cyanobiphenyl components. The simulations show also that both parallel and anti-parallel dipole correlation take place in E7 but that the strong anti-parallel dipole correlation is localised to particular arrangements of molecules. It is possible to identify two key preferred configurations for molecular pairs in the fluid, which explain the form of the dipole correlation function, g(1)(r).