Ultrahigh-pressure phases of H2O ice predicted using an adaptive genetic algorithm

We propose three phases of H2O under ultrahigh pressure. Our structural search was performed using an adaptive genetic algorithm which allows an extensive exploration of crystal structure at density functional theory accuracy. The sequence of pressure-induced transitions beyond ice X at 0 K should be ice X -> Pbcm -> Pbca -> Pmc2(1) -> P2(1) -> P2(1)/c phases. Across the Pmc2(1)-P2(1) 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.