Can gas hydrate structures be described using classical simulations?

Abstract

Quantum path-integral simulations of the hydrate solid structures 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 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 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 at temperatures above 150 K and the relative stabilities between the hydrates and ice I(h). 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, I(h)-->sII-->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 150 K, the sublimation energies, the constant pressure heat capacity, and the radial distribution functions, the incorporation of nuclear quantum effects is indeed required.

A theoretical study of the confinement of methane in water clusters

“…This potential was used in Monte Carlo searches of the optimal isomers. We also used another united‐atom model for methane22 in combination with the TIP4P/200520 and TIP4P/Ice21 models of water, that have recently been used in simulations of the hydrate‐liquid water–gas coexistence 18…”

“…While numerical simulations have been used to study the hydrate‐liquid water–gas coexistence18 and even the spontaneous nucleation,19 the reliability of the results depends on the model analytical potentials that have been used, which have been empirical models with parameters fitted to reproduce experimental data of liquid water20 and ice Ih,21 and of the excess chemical potential of methane in aqueous solution 22. Instead, a purely theoretical approach can be used to test the ability of the empirical models to correctly describe the assembly of water molecules around methane and other solutes.…”

Molecular dynamics simulation of CO2 hydrates: Prediction of three phase coexistence line

“…As another combination of both approaches, recently Velaga and Anderson 31 used preliminary quantum calculations to parameterize an intermolecular model and describe CO 2 hydrate phase equilibria. The influence of nuclear quantum effects on hydrate structures has been also considered by Conde et al 32 Nevertheless, CO 2 hydrates phase equilibrium calculations using MS are scarce. An important element in hydrates global phase diagram, in a PT projection, is the three phase line.…”

Path-integral simulation of solids

“…These quantum effects were rationalized in terms of the degree of tetrahedral ordering in the different ice polymorphs and the volume variation associated to each phase transition 180 . An extension to other phases of water consisted in studying hydrate solid structures 181 . A classical description of hydrates is able to predict correctly the densities at T > 150 K and the relative stabilities between the hydrates and ice Ih.…”