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It has been speculated that small guest molecules such as Ar, Kr, N[sub 2], and O[sub 2] may doubly occupy the large cages of the type II structure of clathrate hydrate. In this study we examined this possibility on Ar hydrate with molecular dynamics calculations. It was found that the total energy and pressure of the system increased significantly when the large cages were doubly occupied. Furthermore, even though Ar clathrate with double occupancy may be stabilized by external pressure, the cubic structure is severely distorted. Owing to restricted empty space, the motions of the Ar atoms in doubly occupied large cages are highly correlated and give a unique stretch vibrational band that can be used for characterization. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the use of molecular dynamics simulation to obtain a complete set of vibrational spectra for proton ordered ice XI. Use of Kumagai, Kawamura and Yokokawa potential model allowing unconstrained atomic motions; Comparison between the power of ice Ih and XI; Observation of spectral properties in the atom velocities polarized spectra.

We carried out molecular dynamics simulations of interstitial water molecules in ice to clarify behavior and vibrational properties of self-interstitials in ice. We used an atom–atom potential model developed by Kumagai et al. [N. Kumagai, K. Kawamura and T. Yokokawa, Mol. Sim. 12, 177 (1994)] which allows intramolecular motions. We confirmed high reliability of the potential model for reproducing the structure and vibrational spectra of ice. From trajectory observations of self-interstitials, we found that a stable interstitial site is an uncapped trigonal site. By comparing power spectra of vibrational motions for self-interstitials with that of matrix ice, we found that the large formation entropy of the interstitial molecule is attributed to much higher density of states in a low frequency region for the interstitial than that for the matrix. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Low-frequency modes of gas hydrates with Xe, Ar, O[SUB2], and N[SUB2] atoms/molecules have been studied by inelastic neutron-scattering and molecular dynamics simulations. Type I and type II clathrate hydrates show some small but significant differences of the low-frequency host contribution to the density of states. Both differ markedly from that of ice Ih and depend only weakly on the guest. The vibrational modes associated with Xe atoms were observed at 2.2, 2.9, and 4.0 meV (T = 100 K). They are in good agreement with predictions from molecular dynamics simulations. In the case of N[SUB2] hydrate we found a well-defined peak at about 2 meV, which shows a remarkable shift to higher frequency with increasing temperature. This peak and a broad peak that is overlapped with the host lattice modes (6.5 and 10.5 meV) are assigned to the vibration of N[SUB2] molecules in the large and small cages, respectively. The calculated vibrational spectra of N[SUB2] molecules in doubly occupied large cages show a significant distinct spectral distribution. The anharmonic shift of the guest atoms in large cages is significantly less pronounced in Xe hydrates suggesting that guest-host interactions can vary considerably from one system to another. [ABSTRACT FROM AUTHOR]