  We propose three new phases of H2O under ultrahigh pressure. Our structural
search was performed using an adaptive genetic algorithm which allows an
extensive exploration of crystal structure. The new sequence of
pressure-induced transitions beyond ice X at 0 K should be ice X - Pbcm - Pbca
- Pmc21 - P21 - P21/c phases. Across the Pmc21 - P21 transition, the
coordination number of oxygen increases from 4 to 5 with a significant increase
of density. All stable crystalline phases have nonmetallic band structures up
to 7 TPa.
