Molecular-dynamics study of structure II hydrogen clathrates

Abstract

Molecular-dynamics simulations are used to study the stability of structure II hydrogen clathrates with different H2 guest occupancies. Simulations are done at pressures of 2.5 kbars and 1.013 bars and for temperatures ranging from 100 to 250 K. For a structure II unit cell with 136 water molecules, H2 guest molecule occupancies of 0-64 are studied with uniform occupancies among each type of cage. The simulations show that at 100 K and 2.5 kbars, the most stable configurations have single occupancy in the small cages and quadruple occupancy in the large cages. The optimum occupancy for the large cages decreases as the temperature is raised. Double occupancy in the small cages increases the energy of the structures and causes tetragonal distortion in the unit cell. The spatial distribution of the hydrogen guest molecules in the cages is determined by studying the guest-water and guest-guest radial distribution functions at various temperatures.

The thermodynamic and ground state properties of the TIP4P water octamer

“…The TIP4P model is a four-site model for water, therefore one would anticipate significant differences in the thermodynamic properties when comparing with the SPC model for water. Given the difficulties encountered while trying to resolve which hydrogen bonding arrangement is favored energetically for the real water octamer by potential models, it is highly likely that future simulations of more complicated systems, such as clathrates for example, [54][55][56][57][58][59][60][61][62] will require reproducibility tests of quantitative results with two or more models. The classical heat capacity of the octamer has proved to be sensitive to the details of the potential energy surface.…”

“…As such, the water octamer serves as an ideal testbed for theories and numerical methods for the simulation of more complex systems that contain water. [54][55][56][57][58][59][60][61][62] Despite the relatively small size of the water octamer, its global optimization has proved to be challenging. Stillinger and David 3 report the first optimized structure of the octamer using a model that includes polarization corrections.…”

Proton disorder and the dielectric constant of type II clathrate hydrates

“…As with the single occupancy results that we have reported, we have used the same potential models used in Ref. 22. Over the time scales of our simulations, we find the lattice to be unstable with respect to dissociation at 2 kbar pressure and temperatures as low as 150 K. Whether the instability is a consequence of the model or the physics of the real system is unknown.…”

“…Such inclusion compounds are known as clathrate hydrates, 1 and the clathrate cages principally take on one of three lattices often called sI, sII, and sH with more complex structures observed 2,3 especially for mixed systems. 4 Methane, [5][6][7][8][9][10][11][12] nitrogen, [13][14][15][16][17] argon, 18 hydrogen, [19][20][21][22][23][24][25][26] and hydrocarbons 27 are among the important gas phase species that can form stable clathrate hydrates.…”

Communication: Librational dynamics in water, sI and sII clathrate hydrates, and ice Ih: Molecular-dynamics insights

“…39 These box lengths resulted in pressures near ambient for liquid water, sI hydrate, and ice, and ∼100 bars for sII hydrate. To ascertain guest effects on lattice vibrational motion, we performed comparative MD of sI and sII containing a rigid H 2 model, 40 40 We determined essentially no effect nor any appreciable pressure difference, owing to little lattice distortion by H 2 . INS data 17 were obtained featuring Xe guests for sI, and deuterated tetrahydrofuran for sII, making direct MD-versus-INS comparison more challenging (vide supra).…”