Abstract
The normal modes of ice IX were investigated using the CASTEP code package, which is based on density functional theory. We found that the translational modes could be divided into three categories: four-bond vibrations, which possessed the highest energy; two-bond vibrations, which possessed the medium energy; and cluster vibrations with the lowest energy. The former two categories represent monomers vibrating against neighbors and present as two distinct peaks in many ice phases recorded in inelastic neutron-scattering experiments. It is typically difficult to assign the molecular vibration peaks in the far infrared region. The method we developed to analyze the normal modes, especially in the translation band of ice IX, provided physical insights into the vibrational spectrum of ice.