Over the last three years, Burkhard Militzer's group has been working on understanding water at megabar pressures with ab initio computer simulations. In the interiors of Uranus and Neptune (dashed lines in figure) where such pressures exist, water is predicted to occur in a superionic state where the oxgyen atoms remain stationary like in a solid while the hydrogen atoms diffuse throughout the crystal like a fluid. In the most recent article that appeared in the journal Physical Review Letters, postdoc Hugh Wilson, summer student Michael Wong, and Burkhard Militzer, show that, at 1.0±0.5 megabars, the oxygen sub-lattice in superionic water changes from a body-centered cubic lattice to an face-cented cubic lattice (inset). This transformation lead to a more efficient packing but also reduces the diffusion rate of the hydrogen atoms, which may have further implications for electronic conductivity and magnetic dynamo in Uranus and Neptune. This theorectical prediction is expected to be verified with laboratory experiments using shock wave and x-ray diffration techniques.