AB-INITIO TOTAL-ENERGY STUDIES OF THE STATIC AND DYNAMICAL PROPERTIESOF ICE IH

We present a detailed study of the structural and dynamical properties of ice lh. Methods used include the ab-initio pseudopotential and mol ecular orbital techniques. In order to simulate the proton disorder pr esent in ice Ih supercells are constructed with the orientation of wat er molecules determined in such a way that all possible nearest neighb or orientations are included while still obeying the Bernal-Fowler ice rules. For structures considered all structural parameters are relaxe d. The dependence of bond length on the orientation of surrounding mol ecules in both the dimer and fully coordinated system is discussed. Al l local dimer orientations are accounted for in the supercell of ice I h considered. The dynamical properties of a 16-molecule supercell mode l of ice Ih are then investigated via direct determination of the zone center dynamical matrix. Here the dynamical matrix is calculated dire ctly from atomic forces obtained when each atom is shifted away from i ts equilibrium position. The vibrational modes are then calculated, an d the mode frequency as a function of microscopic deformation is discu ssed.