Evaluation of General and Tailor Made Force Fields via X-ray Thermal Diffuse Scattering Using Molecular Dynamics and Monte Carlo Simulations of Crystalline Aspirin

Abstract

We have performed a comparison of the experimental thermal diffuse scattering (TDS) from crystalline Aspirin (form I) to that calculated from molecular dynamics (MD) simulations based on a variety of general force fields and a tailor-made force field (TMFF). A comparison is also made with Monte Carlo (MC) simulations which use a "harmonic network" approach to describe the intermolecular interactions. These comparisons were based on the hypothesis that TDS could be a useful experimental data in validation of such simulation parameter sets, especially when calculations of dynamical properties (e.g., thermodynamic free energies) from molecular crystals are concerned. Currently such a validation of force field parameters against experimental data is often limited to calculation of specific physical properties, e.g., absolute lattice energies usually at 0 K or heat capacity measurements. TDS harvested from in-house or synchrotron experiments comprises highly detailed structural information representative of the dynamical motions of the crystal lattice. Thus, TDS is a well-suited experimental data-driven means of cross validating theoretical approaches targeted at understanding dynamical properties of crystals. We found from the results of our investigation that the TMFF and COMPASS (from the commercial software "Materials Studio") parameter sets gave the best agreement with experiment. From our homologous MC simulation analysis we are able to show that force constants associated with the molecular torsion angles are likely to be a strong contributing factor for the apparent reason why these aforementioned force fields performed better.

Computational Assessment of the Molecular Structure and Properties for High Energy Density Fuel

“…The calculations performed in this part were done with the Forcite module of Materials Studio 2017 software package . The optimized structures by Gaussian were employed as the input geometries to determine the densities (ρ) using the COMPASS , forcefield with a cutoff distance of 12.5 Å.…”

“…The calculations performed in this part were done with the Forcite module of Materials Studio 2017 software package. 32 The optimized structures by Gaussian were employed as the input geometries to determine the densities (ρ) using the COMPASS 33,34 (NPT) ensemble with the Berendsen barostat method and Anderson thermostat method to control the system pressures and temperatures. The simulation cell contains 128 target hydrocarbons.…”

Generating Cocrystal Polymorphs with Information Entropy Driven by Molecular Dynamics-Based Enhanced Sampling

“…In this class of MD simulations, a trajectory is guided by modifying the original potential energy surface and corresponding forces, by varying the temperature of the system, or both, to facilitate sampling of otherwise rare transitions between local minima. Indeed, it is becoming increasingly recognized that such molecular simulation approaches are an important component of a CSP protocol, − as they subject putative structures to experimental conditions, can check their stability under these conditions, and produce relevant thermal averages at experimentally relevant temperatures and pressures. The need for enhanced-sampling approaches arises from the fact that polymorphs may be separated by high-energy barriers even if the free energy differences between them are small.…”