What is the Optimal Dipole Moment for Nonpolarizable Models of Liquids?

In classical nonpolarizable models, electrostatic interactions are usually described by assigning fixed partial charges to interaction sites. Despite the multitude of methods and theories proposed over the years for partial charge assignment, a fundamental question remains─what is the correct degree of polarization that a fixed-charge model should possess to provide the best balance of interactions (including induction effects) and yield the best description of the potential energy surface of a liquid phase? We address this question by approaching it from two separate and independent viewpoints: the QUantum mechanical BEspoke (QUBE) approach, which assigns bespoke force field parameters for individual molecules from ab initio calculations with minimal empirical fitting, and the Polarization-Consistent Approach (PolCA) force field, based on empirical fitting of force field parameters with an emphasis on transferability by rigorously accounting for polarization effects in the parameterization process. We show that the two approaches yield consistent answers to the above question, namely, that the dipole moment of the model should be approximately halfway between those of the gas and the liquid phase. Crucially, however, the reference liquid-phase dipole needs to be estimated using methods that explicitly consider both mean-field and local contributions to polarization. In particular, continuum dielectric models are inadequate for this purpose because they cannot account for local effects and therefore significantly underestimate the degree of polarization of the molecule. These observations have profound consequences for the development, validation, and testing of nonpolarizable models.

Correction to "New Force-Field for Organosilicon Molecules in the Liquid Phase".

[This corrects the article DOI: 10.1021/acsphyschemau.1c00014.].

New Force-Field for Organosilicon Molecules in the Liquid Phase.

In this paper, we present a new molecular model that can accurately predict thermodynamic liquid state and phase-change properties for organosilicon molecules including several functional groups (alkylsilane, alkoxysilane, siloxane, and silanol). These molecules are of great importance in geological processes, biological systems, and material science, yet no force field currently exists that is widely applicable to organosilicates. The model is parametrized according to the recent Polarization-Consistent Approach (PolCA), which allows for polarization effects to be incorporated into a nonpolarizable model through post facto correction terms and is therefore consistent with previous parametrizations of the PolCA force field. Alkyl groups are described by the United-Atom approach, bond and angle parameters were taken from previous literature studies, dihedral parameters were fitted to new quantum chemical energy profiles, point charges were calculated from quantum chemical optimizations in a continuum solvent, and Lennard-Jones dispersion/repulsion parameters were fitted to match the density and enthalpy of vaporization of a small number of selected compounds. Extensive validation efforts were carried out, after careful collection and curation of experimental data for organosilicates. Overall, the model performed quite well for the density, enthalpy of vaporization, dielectric constant, and self-diffusion coefficient, but it slightly overestimated the magnitude of self-solvation free energies. The modular and transferable nature of the PolCA force field allows for further extensions to other types of silicon-containing compounds.

Polarization Corrections and the Hydration Free Energy of Water.

Classical nonpolarizable water models play a crucial role in computer simulations due to their simplicity and computational efficiency. However, the neglect of explicit polarization can jeopardize their accuracy and predictive capabilities, particularly for properties that involve a change in electrostatic environment (e.g., phase changes). In order to mitigate this intrinsic shortcoming, highly simplified analytical polarization corrections describing the distortion of the molecular dipole are commonly applied in force field development and validation. In this paper, we perform molecular dynamics simulations and thermodynamic integration to show that applying the current state-of-the-art polarization corrections leads to a systematic inability of current nonpolarizable water models to simultaneously predict the experimental enthalpy of vaporization and hydration free energy. We go on to extend existing theories of polarization and combine them with data from recent ab initio molecular dynamics simulations to obtain a better estimate of the real contribution of polarization to phase-change energies and free energies. Our results show that for strongly polar molecules like water, the overall polarization correction is close to zero, resulting from a cancellation of multipole distortion and purely electronic polarization effects. In light of these findings, we suggest that parametrization of classical nonpolarizable models of water should be revisited in an attempt to simultaneously describe phase-change energetics and other thermodynamic and structural properties of the liquid.