Vibrations of Methane in Structure I Clathrate Hydrate—an ab initio Density Functional Molecular Dynamics Study

The stretching vibrations of methane in the small and large cavity of structure I clathrate hydrate have been studied with ab initio molecular dynamics using a linear scaling pseudopotential density functional method. The vibrational density of states, obtained by Fourier transform of the atom velocity autocorrelation function, show complicated profiles caused by symmetry lowering and effects of non-vanishing electrostatic potential inside the clathrate cages. The calculated C–H stretching vibrations are lower than in the free molecule. Furthermore, the vibrational frequencies in the large cage are shifted to lower frequency with respect to the small cage. The theoretical predictions are in complete agreement with experiment. These theoretical findings, along with recent experimental measurements do not reveal local minima inside the cavities that may enhance the guest–host interactions as would have suggested by the ‘loose cage–tight cage’ Charles–Pimentel model.