Molecular dynamics studies of proton ordering effects on lattice vibrations in ice I_h

We carried out molecular dynamics simulations of the proton-disordered ice I_h and the proton-ordered ice XI using a new potential model which allows total degree of freedom of atomic motions. The power spectra calculated from velocity autocorrelation function successfully reproduced the spectral characteristics obtained by inelastic incoherent neutron scattering with relative shifts of the peak frequencies. We found that proton ordering affects the librational modes more than other modes such as the translational and the intramolecular vibrations. A wide broad band of librational modes of the proton-disordered phase splits into four distinct peaks due to the proton-ordering transition from I_h to XI.