First principles lattice dynamics studies of the vibrational spectra of ice

We present studies of the vibrational properties of a number of ice structu res as evaluated by 'first principles' lattice dynamics. Calculations are p erformed within the generalized gradient approximation to density functiona l theory using the 'ab initio' pseudopotential method. Dynamical properties are determined within the harmonic approximation by finite difference eval uation of the dynamical matrix from atomic forces. The resulting normal mod es of vibration are analyzed in detail by projection onto pure intra- and i nter-molecular modes. The importance of configurational disorder is assesse d by comparison of results from different water molecule orientations in si milar supercells. All structures considered here comprise of tetrahedral hy drogen-bonded networks and include low-pressure hexagonal and cubic structu res and high-pressure cubic structures. Calculated vibrational density of s tates are compared with results from quasielastic neutron scattering and th e microscopic origin of various features in the spectra discussed. (C) 1999 Elsevier Science B.V. All rights reserved.