Abstract
In this work, we propose an improved QM/MM-based strategy to determine condensed-phase polarizabilities and we use this approach to optimize a new and simple polarizable four-site water model for classical molecular simulation. For the determination of the model value for the polarizability from QM/MM, we show that our proposed consensus-fitting strategy significantly reduces the uncertainty in calculated polarizabilities in cases where the size of the local external electric field is small. By fitting electrostatic, polarization and dispersion properties of our water model based on quantum and/or combined QM/MM calculations, only a single model parameter (describing exchange repulsion) is left for empirical calibration. The resulting model performs well in describing relevant pure-liquid thermodynamic and transport properties, which illustrates the merit of our approach to minimize the number of free variables in our model.
Highlights
The presence of water is vital for biomolecular action and cellular function [1,2]
First-generation water models commonly used in molecular simulation have three interaction sites, with static partial charges located on the oxygen and both hydrogens [9,10] and with dispersion and exchange interactions handled by a single van der Waals site per water molecule
We propose here a constrained-fitting strategy and demonstrate that our redesigned approach allows for determining a consensus value for the polarizability based on our Quantum Mechanical (QM)/molecular mechanics (MM) calculations
Summary
The presence of water is vital for biomolecular action and cellular function [1,2]. It is necessary to simulate systems in a water model that can accurately mimic solvent environmental effects when studying e.g., protein dynamics and ligand binding [3,4,5]. We showed that, by introducing a higher-order dispersion term (C8 , which can contribute up to one third of molecular dispersion interactions) and by using atoms-in-molecules (AIM) quantum calculations of Exchange-Hole-Dipole moments (XDM) to determine van der Waals parameters [23], we obtained an alkane model that reproduces pure-liquid thermodynamic properties within a few percent without further parameter calibration [24]. Together with the other parameters derived from quantum (and XDM) calculations, this leaves us with a single parameter (i.e., the repulsive van der Waals constant) to be empirically calibrated in order to obtain our final water model We calibrate this parameter based on pure-liquid thermodynamic properties of water at ambient conditions, and we find that our final model has a static dielectric permittivity at room temperature and heat of vaporization at a wide range of temperatures (250–370 K). That are close to experimental estimates, while it shows a maximum in water density in the expected region
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