MECHANICAL AND MOLECULAR-PROPERTIES OF ICE-VIII FROM CRYSTAL-ORBITAL AB-INITIO CALCULATIONS

The proton-ordered ice VIII structure has been investigated by ab init io periodic Hartree-Fock calculations in the pressure interval from 0 to 30 GPa using a 6-31G* basis set, The structure was optimized by en ergy-minimization at different volumes, and from the resulting energy vs volume relationship; the equation of state of ice VIII was derived; The variation of the structure,intramolecular geometry, Mulliken char ges, electron density, Raman spectrum, and infrared stretching vibrati ons with varying pressure were investigated. The agreement with existi ng experimental data is generally good. Nearest-neighbor hydrogen-bond ed O...O distances decrease from 2.88 to 2.57 Angstrom as the pressure is increased from 0 to 30 GPa. For the same: pressure range, the intr amolecular OH bond increases from 0.951 to 0.955 Angstrom (giving a dr (OH)/dP value of 0.000 14 Angstrom/GPA), the Mulliken charge on H incr eases from +0.386 to +0.452, the calculated bulk modulus increases fro m similar to 25 to similar to 160 GPa), (corresponding experimental va lues ire similar to 25 at 2.4 GPa and similar to 135 at 30 GPa), and t he electron density redistribution is considerably enhanced. The frequ ency downshift of the OH stretching vibration varies from -200 cm(-1) at 2.4 GPa to -500 cm(-1) at 20 GPa; the corresponding experimental va lues are -300 and -650 cm(-1). Electronic density-of-states diagrams a re presented.