Ground-state structures of ice at high pressures fromab initiorandom structure searching

Ab initio random structure searching based on density functional theory is used to determine the ground-state structures of ice at high pressures. Including estimates of lattice zero-point energies, ice is predicted to adopt at least three crystal phases beyond $Pbcm$. The underlying sublattice of O atoms remains similar among them, and the transitions can be characterized by reorganizations of the hydrogen bonds. The symmetric hydrogen bonds of ice X and $Pbcm$ are initially lost as ice transforms to structures with symmetries Pmc2₁ (800--950 GPa) and P2₁ ($1.17$ TPa), but they are eventually regained at $5.62$ TPa in a layered structure $C2/m$. The P2₁→C2/m transformation also marks the insulator-to-metal transition in ice, which occurs at a significantly higher pressure than recently predicted.