Building Water Models, A Different Approach

TL;DR

This paper introduces a novel water model, OPC, optimized through an analytical approach to better replicate bulk water properties and hydration energies, surpassing traditional models in accuracy and temperature range.

Contribution

The paper presents a new point charge water model optimized via analytical equations, improving accuracy over existing models without geometric constraints.

Findings

OPC model achieves 0.76% average error relative to experiment.

OPC accurately predicts hydration free energies with RMS error < 1 kcal/mol.

Model maintains close agreement with experimental data across various temperatures.

Abstract

Simplified, classical models of water are an integral part of atomistic molecular simulations, especially in biology and chemistry where hydration effects are critical. Yet, despite several decades of effort, these models are still far from perfect. Presented here is an alternative approach to constructing point charge water models - currently, the most commonly used type. In contrast to the conventional approach, we do not impose any geometry constraints on the model other than symmetry. Instead, we optimize the distribution of point charges to best describe the "electrostatics" of the water molecule, which is key to many unusual properties of liquid water. The search for the optimal charge distribution is performed in 2D parameter space of key lowest multipole moments of the model, to find best fit to a small set of bulk water properties at room temperature. A virtually exhaustive search is enabled via analytical equations that relate the charge distribution to the multipole moments. The resulting "optimal" 3-charge, 4-point rigid water model (OPC) reproduces a comprehensive set of bulk water properties significantly more accurately than commonly used rigid models: average error relative to experiment is 0.76%. Close agreement with experiment holds over a wide range of temperatures, well outside the ambient conditions at which the fit to experiment was performed. The improvements in the proposed water model extend beyond bulk properties: compared to the common rigid models, predicted hydration free energies of small molecules in OPC water are uniformly closer to experiment, root-mean-square error < 1kcal/mol.