A refined general AMBER force field for liquid acetonitrile: Development and validation.

Development of United-Atom Force Fields for Monomeric and Oligomeric Ionic Liquids through Regression-Guided Optimization of Electronic Continuum Correction Parameters

Ionic liquids are salts that exist in the liquid state at room temperature and exhibit high viscosity because of their strong electrostatic interactions. It was difficult to reproduce their viscosity by molecular dynamics simulations with conventional nonpolarizable force fields; however, recent development of force fields implementing electronic continuum correction (ECC), which accounts for polarizability, has enabled accurate predictions. Here, we present a regression-guided strategy to optimize scaling factors for ECC charges and Lennard-Jones parameters for monomeric (BMIM+, MOEMIM+) and oligomeric (IL22+, IL44+) imidazolium-based cations paired with TFSI– with united-atom models. The scaling factors were optimized to simultaneously reproduce experimental density and viscosity. To validate the strategy, we calculated the temperature dependence of density, diffusion coefficient, conductivity, and viscosity of BMIM–TFSI, achieving good agreements with experiments. Moreover, scaling factors optimized for MOEMIM+, which shares similar chemical structure and elemental compositions with IL22+ and IL44+, were found to be transferable to these compounds. Thus, this work not only provides a practical regression-guided workflow for selecting ECC-based united-atom force-field parameters but also suggests their transferability across chemically related ionic liquids.

HessFit: Deriving Molecule-Specific Force Fields from Quantum Mechanical Hessians.

Tracking down the structural and enthalpic changes in molecular materials upon melting.

A novel X10 benchmark set is introduced to enable an assessment of the computational performance of atomistic simulations of enthalpies of fusion for molecular materials. The fusion enthalpy, i.e. the enthalpy of melting, represents a fundamental descriptor of the solid-liquid equilibrium, and as such, the ability to predict this property is highly valuable across various fields of material design. Computational accuracy is tested for all-atom molecular-dynamics simulations, relying either on a classical generic non-polarizable force-field model (MD), or on a selected density-functional-theory (DFT) treatment of the electronic degrees of freedom in ab initio molecular dynamics (AIMD). Performance of these MD and AIMD methods is critically assessed against reference experimental data and it is evaluated in terms of the fusion enthalpies and related structural descriptors, such as bulk phase densities. A detailed interpretation of the predicted fusion enthalpies in terms of important cohesive non-covalent interactions, such as hydrogen bonding and dispersion forces, and variations of their intensity upon melting is presented. A special emphasis is laid on investigations of the finite-size artifacts that are necessarily at play in costly AIMD simulations that are feasible only for small molecular ensembles. The presented results indicate that the fusion enthalpies for neutral-molecular materials can be typically predicted within roughly 2 kJ mol-1 from the experiment with the PBE-D3(BJ)/GTH based AIMD model, yielding a somewhat higher accuracy on average than the classical force-field based simulations. This finding is important as similar DFT theories are commonly used nowadays as a source of training data for development of machine-learning interatomic potentials.

A Gradient-Corrected Polarizable Continuum Model for Improved Reaction Fields at the SCF Level.

We propose an improved model, termed the gradient-corrected PCM (GCPCM), for improving the energy accuracy of the polarizable continuum model (PCM). Our previous study revealed deficiencies of PCM in describing the reaction field, i.e., the electrostatic potential generated by the solvent. These deficiencies can be partially alleviated by introducing an empirical correction to the solvent charges. As a result, solute-solvent interactions are improved at the self-consistent field level, leading to enhanced energy accuracy. The performance of GCPCM was evaluated through single-point calculations and geometry optimizations of phenol and phenolate, calculations of the free energy profile for proton transfer in glycine, and analysis of solvent responses of the HOMO and LUMO orbital energies of Brooker's merocyanine. The results demonstrate that the characteristic destabilization of charged solutes observed in conventional PCM is effectively resolved. Furthermore, despite having a computational cost comparable to that of PCM, GCPCM shows the potential to achieve an energy accuracy similar to that of 3D-RISM-SCF. The development of GCPCM enables more convenient and accurate treatment of solvation effects, which is expected to allow researchers to focus on other important challenges, such as the accurate description of electronic states.

Polarizable Split-Charge Equilibration for Accurate Modeling of Nonbonded Interactions

Accurate modeling of nonbonded interactions remains challenging because conventional force fields often provide inadequate descriptions of polarization and charge transfer. Here, we introduce a polarizable split-charge equilibration (PSQ) framework that combines localized charge flow with dipolar polarization, thereby mitigating the unphysical long-range charge delocalization associated with polarizable charge-equilibration models. To enable practical large-scale simulations, we develop an efficient PSQ implementation through algorithmic optimization and parallelization, achieving performance comparable to conventional approaches despite the model’s greater formal complexity. We combine PSQ with the universal nonbond potential for van der Waals interactions and an explicit four-body hydrogen-bond term to form the PUxH framework. PUxH achieves near-chemical accuracy in predicting small-molecular pair interactions and accurately reproduces the bulk properties of water, ammonia, and benzene. Overall, PUxH provides a transferable, physically grounded, and computationally efficient framework for modeling nonbonded interactions in molecular and condensed-phase systems.

On calculating polar solvation energy of nonrigid proteins in the Poisson-Boltzmann theory

The generalized PB theory is validated using a perturbed two-atom system and a diverse set of proteins with different structures in vacuum and water, demonstrating its accuracy and robustness, regardless of the choice of sharp-interface and diffuse-interface PB models and different numerical solvers.