Can gas hydrate structures be described using classical simulations?

Abstract

Quantum path-integral simulations of the gas hydrate solids have been performed using the recently proposed TIP4PQ/2005 model. By also performing classical simulations using this model, the impact of the nuclear quantum effects on the hydrates is highlighted; nuclear quantum effects significantly modify the structure, densities and energies of the gas hydrates, leading to the conclusion that nuclear quantum effects are important not only when studying the solid phases of water but also when studying the gas hydrates. To analyze the validity of a classical description of hydrates a comparison of the results of the TIP4P/2005 model (optimized for classical simulations) with those of TIP4PQ/2005 (optimized for path integral simulations) was undertaken. A classical description of hydrates is able to correctly predict the densities of the hydrates at temperatures above 150K, and the relative stabilities between the hydrates and ice Ih. The inclusion of nuclear quantum effects does not significantly modify the sequence of phases found in the phase diagram of water at negative pressures, namely Ih to sII to sH. In fact the transition pressures are little affected by the inclusion of nuclear quantum effects; the phase diagram predictions for hydrates can be performed with reasonable accuracy using classical simulations. However, for a reliable calculation of the densities below 150K, the sublimation energies, the constant pressure heat capacity and the radial distribution functions, the incorporation of nuclear quantum effects is indeed required.