Ultrahigh-pressure phases of H ₂O ice predicted using an adaptive genetic algorithm

We propose three phases of H ₂O 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.