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

Abstract

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.

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

“…Such attributes position QMD-FFs as a method of choice to rationalize specific structure–property relationships, accurately predict the macroscopic behavior of supra-molecular assemblies, stimulate new experiments, and eventually bring us closer to a de novo design of programmed assembly based on solely the molecular structure. Unfortunately, fulfilling such potential has remained an elusive task. − In particular, large-scale supra-molecular reorganization processes, where single-molecule events are intertwined with slow (>100 ns), thermally driven collective dynamics have never been investigated with QMD-FFs. In fact, even in the most recent applications reported in the literature, the observation time was limited to few nanoseconds of simulation. , …”

“…In this work, we show that QMD-FF provides a truly multiscale description of the complex self-assembly process that leads to an orientationally ordered phase upon cooling in ambient conditions with a remarkable accuracy and, to the best of our knowledge, the first atomically detailed picture of a self-reorganization processes spanning over 200 ns. To this end, we adopt a specific and accurate QMD-FF, recently parametrized according to the Joyce / Picky procedure, − for the 4′- n -pentyl-4-cyanobiphenyl (5CB), a well-known benchmark liquid-crystalline molecule. The results recently achieved therein suggested that the dramatic failure of general-purpose FFs, which predict a spontaneous assembly at temperatures more than 120 °C higher than the experiment, , could be significantly corrected by QMD-FFs.…”

“…The results recently achieved therein suggested that the dramatic failure of general-purpose FFs, which predict a spontaneous assembly at temperatures more than 120 °C higher than the experiment, , could be significantly corrected by QMD-FFs. Yet, despite being promising, the MD outcomes reported in ref did not allow for a detailed characterization of the supramolecular architecture or for an atomistic insight into the processes possibly leading to the orientational reorganization, thus calling for a further, more extended investigation. Liquid crystals (LCs) are indeed the ideal candidate to test QMD-FFs reliability for self-assembled materials.…”

“…In conclusion, we have shown that by using an “ ab initio ” based approach we can obtain a truly multiscale description of complex self-assembly processes, as for instance the temperature-driven reorganization of an isotropic liquid to an orientationally ordered nematic phase. Concretely, the results obtained with MD runs employing an accurate QMD-FF, specifically tailored for the 5CB nematogen, were found in a noticeable quantitative agreement with a plethora of experiments characterizing different stages of this process. Furthermore, the MD capability to bridge several time scales, of utmost relevance yet often absent in experimental ensemble averages, was exploited to gain a picosecond detailed atomic understanding of the self-assembly processes and associated instabilities, which is corroborated by the reliability of the description achieved for the slow collective reorientation.…”

Complexes of Alkaline and Ammonium Cations with Dopamine and Eumelanin Precursors: Dissecting the Role of Noncovalent Cation−π and Cation–Lone Pair (σ-Type) Interactions

“…The aim of such detailed 2D maps is 2-fold. First, they can serve as reference to tune for instance quantum mechanically derived force fields, − which in turn can be employed in classical MD simulations, thus taking into account larger systems and other fundamental ingredients as solvent, other ionic species or embedding, to reliably describe supramolecular phenomena . Next, the wide exploration of the Δ E multidimensional surface, achieved through the analysis of multiple landscapes, allows for detecting additional interactions that might be overlooked in approaches considering few conformers or even in more limited scans.…”