Assessing the Importance of the H2-H2O-H2O Three-Body Interaction on the Vibrational Frequency Shift of H2 in the sII Clathrate Hydrate and Comparison with Experiment.

Abstract

The vibrational frequency shift of H2 in the 512 cage of the sII clathrate hydrate with and without surrounding water molecules is reported at 0 K, using diffusion Monte Carlo calculations for the ground and first excited vibrational states of H2. Approximate 1d calculations of the frequency shift are also reported with the H2 at the equilibrium position in the clathrate hydrate. These calculations make use of full-dimensional potential energy surfaces for the H2-H2O 2-body and H2-H2O-H2O 3-body interactions. The inclusion of the 3-body interaction is shown to make roughly a 33% contribution to the frequency shift and to bring the calculated value of -40 ± 4 cm-1 to within just 3 cm-1 of the experimental value at 20 K. This level of agreement with experiment may be somewhat fortuitous; however, the importance of the 3-body interaction is firmly established by these calculations. The frequency shift reported here with 2-body interactions does not agree with a previously reported calculation using just 2-body interactions from a different ab initio potential energy surface and with a different method to obtain the frequency shift. A similar 1d calculation of the frequency shift using that potential is reported and agrees to within roughly 10% of the one previously reported. Therefore, this suggests that the difference between the present calculations and the previous one using just 2-body interactions is mainly due to differences in the potential energy surfaces.