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This paper introduces a novel simulation technique combining Lattice Switch Monte Carlo and thermodynamic integration to compute free energy differences between clathrate hydrate structures with different stoichiometries. The method allows for the determination of coexistence parameters by measuring free energy differences between fully occupied and fully empty hydrate structures. Applying this approach to argon and methane hydrates, the study achieves coexistence pressure predictions that align well with experimental data.
Accurately predicting the stability and coexistence of different clathrate hydrate structures is now possible thanks to a new simulation technique that bridges Lattice Switch Monte Carlo and thermodynamic integration.
We introduce a simulation technique to compute the free energy difference between two hydrate structures of different stoichiometry connected to a reservoir of gas molecules at a prescribed pressure. The method permits the determination of coexistence parameters for the system when the two hydrate structures have the same number of water molecules $N_w$. The approach is based on performing isobaric Lattice Switch Monte Carlo simulations to measure free energy differences between the hydrate structures when they are either fully occupied by gas molecules, or fully empty. This measurement is combined with thermodynamic integration within an ensemble in which the number of guest molecules $N_g$ can fluctuate under the control of a chemical potential $\mu_g$. We analyze the properties of the resulting constant-$N_w,\mu_g,P,T$ ensemble and show how it can be used to calculate coexistence points via a thermodynamic cycle. Applying the method to argon and methane structures, we find coexistence pressures that are in good agreement overall with the available experimental data.