Application of the independent molecule model to elucidate the dynamics of structure I methane hydrate: A third report

Two new model systems of methane hydrate, larger than the previous systems, are constructed. One consists of 63 small and large cages with a small cage at the centre. The other has 65 small and large cages with a large cage at the centre. Three different H-bonding network patterns between water are formed, and three random orientations of methane in each cage are chosen. Using the surface water fixed method, we obtained the energy minimum conformations, fitted to the X-ray crystallographic structure. With normal mode analysis, we calculated frequencies of 2915.1 cm−1 for a small cage, and 2911.0 cm−1 for a large cage. These frequencies are a little nearer to the Raman spectra than were previous model systems. Treating three force constants of anharmonic potential energy and the strength of H-bonding between methane and water as four parameters, we obtained frequencies of 2913.6 cm−1 for a small cage, a little lower than the Raman, and 2906.6 cm−1 for a large cage, a little higher than the Raman. The calculations thus almost reach the Raman spectra.