Towards a dissociative SPC-like water model - probing the impact of intramolecular Coulombic contributions.

Abstract

The dissociative water potential introduced by Garofalini et al. proved to be a simple and effective description to account for proton transfer in aqueous media, enabling for instance the execution of simulation studies at different pH values. In this model the charge of each particle is represented by a point-charge surrounded by a Gaussian charge-cloud of opposite sign, thus four Coulombic terms (point-charge-point-charge, point-charge-charge-cloud, charge-cloud-point-charge and charge-cloud-charge-cloud) are required per atom pair. In this work it is demonstrated that the Gaussian charge distributions can be removed from the model after a minor modification of the overall atomic point-charges. Despite this substantial modification of the model, structural properties obtained via pair- and angle distributions remain largely unaffected and the change in dynamical properties (vibrational frequencies, proton transfer properties) was found to be minor. As an additional improvement an adjustment of the bending mode vibration was carried out by carefully evaluating the parametrization of the three-body interaction potential, thereby retaining the good agreement of the transfer properties of the aqueous excess proton reported in an earlier study. The proposed simple point-charge (SPC) type parametrization of the Coulombic interactions not only leads to a notable decrease in computational demand but generalizes the dissociative model by improving its transferability to established third-party simulation software and enabling the application of different theoretical approaches such as Ewald summation techniques not considered in the original parametrization. The outlined optimization strategy demonstrates that despite the complex and challenging formulation of the force field, various dynamical properties can be selectively adjusted without influencing other critical parameters of the simulated systems.