  We report a detailed ab initio investigation on hydrogen bonding, geometry,
electronic structure, and lattice dynamics of ice under a large high pressure
range, including the ice X phase (55-380GPa), the previous theoretically
proposed higher-pressure phase ice XIIIM (Refs. 1-2) (380GPa), ice XV (a new
structure we derived from ice XIIIM) (300-380GPa), as well as the ambient
pressure low-temperature phase ice XI. Different from many other materials, the
band gap of ice X is found to be increasing linearly with pressure from 55GPa
up to 290GPa, the electronic density of states (DOS) shows that the valence
bands have a tendency of red shift (move to lower energies) referring to the
Fermi energy while the conduction bands have a blue shift (move to higher
energies). This behavior is interpreted as the high pressure induced change of
s-p charge transfers between hydrogen and oxygen. It is found that ice X exists
in the pressure range from 75GPa to about 290GPa. Beyond 300GPa, a new
hydrogen-bonding structure with 50% hydrogen atoms in symmetric positions in
O-H-O bonds and the other half being asymmetric, ice XV, is identified. The
physical mechanism for this broken symmetry in hydrogen bonding is revealed.
