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Benchmarking the performance of density functional theory and point charge force fields in their description of sI methane hydrate against diffusion Monte Carlo
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71.As the experimental number is a standard enthalpy of dissociation we should not expect quantitative agreement with the DMC dissociation energy, which is a total energy difference. Aside from the temperature/pressure effects present in experiment, there is also the issue of non-stoichiometry (the experimental data of Handa70 was obtained for a methane occupancy of water cages of ca. 96%), which means that configurational entropy is likely to be important for the experimental dissociation enthalpy. This comparison is made simply to show that the number obtained with DMC is reasonable. In fact, analysis of numerous experimental data sets using the Clapeyron equation yields an enthalpy of dissociation of 157 ± 6 meV/CH4 at 150 K and 0.0564 bar.81 This is arguably a better comparison to the zero temperature/pressure DMC calculations and indeed improves agreement, but one should nevertheless exercise caution when comparing a calculated dissociation energy to an experimental enthalpy.
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High quality reference data from diffusion Monte Carlo calculations are presented for bulk sI methane hydrate, a complex crystal exhibiting both hydrogen-bond and dispersion dominated interactions. The performance of some commonly used exchange-correlation functionals and all-atom point charge force fields is evaluated. Our results show that none of the exchange-correlation functionals tested are sufficient to describe both the energetics and the structure of methane hydrate accurately, while the point charge force fields perform badly in their description of the cohesive energy but fair well for the dissociation energetics. By comparing to ice , we show that a good prediction of the volume and cohesive energies for the hydrate relies primarily on an accurate description of the hydrogen bonded water framework, but that to correctly predict stability of the hydrate with respect to dissociation to ice and methane gas, accuracy in the water-methane interaction is also required. Our results highlight the difficulty that density functional theory faces in describing both the hydrogen bonded water framework and the dispersion bound methane.
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