Far-infrared spectrum of ice Ih: A first-principles study

The dielectric properties of ice Ih in the translational far-infrared region (0-400 cm(-1)) have been investigated by means of density functional theory methods. We show that interpretation of the experimental data for the absorption coefficient requires a full account of proton disorder, an ab initio description of the electronic response, and a proper treatment of the longitudinal-optical-transverse-optical (LO-TO) splitting. The orientational average for the LO-TO splitting yields a broad range of values, and the average over proton disorder gives rise to a peak at 150 cm(-1) in the absorption spectrum of ice Ih. The peak is not present in ice XI, the ferroelectric state with lowest energy. Our study supports the notion that the far-IR dielectric properties of water aggregates are tightly related to local molecular order.