Molecular Dynamics Simulations of the Frequency Dielectric Response of Water

Abstract

The dielectric response of water is of great importance, not only due to the general abundance of water, but also because of the importance of water in countless industrial, environmental, and biological functions. Molecular dynamics simulations allow for investigation of this response on a molecular level in a specified environment. However, any molecular dynamics simulation is limited by the models utilised. In this work, the influence of external oscillating electric fields, ranging in frequency from 2 - 16000 GHz, on particular water models was investigated using molecular dynamics simulations and compared to experimental data of real water. Four water models, SPC/E, SPC/ε, SPC/ε₁ and SPC/fw, were investigated for the accuracy of each model's frequency dielectric response, and the ability to reproduce dynamics which contribute to the dielectric response at high frequencies. All simulations were conducted in a constant temperature of 300 K and under a constant pressure of 1 atmosphere. The SPC/fw model is more complex than the other three models in that it allows for flexible bond lengths and angles. Consequently, better agreement with experiment at high frequencies was hypothesised. The SPC/E and SPC/fw models were accurate in reproducing the low frequency contribution to the dielectric function, the main Debye peak, that is characteristic of water. All four models reproduced the dynamics of the very high frequency dielectric response, which is attributed to libration modes, with SPC/fw most closely reproducing experimental results. However, none of the models accurately reproduced the high frequency contributions to the dielectric function, which are the main contributors to the deviation from Debye behaviour over the high GHz and low THz regions.