Fig. 3: Free energy of the X–Pbcm transition.
From: Thermodynamics of high-pressure ice phases explored with atomistic simulations
a Configurations of Pbcm and X. The orthorhombic simulation cell contains 64 water molecules and is commensurate with both the X and the Pbcm structures. Free-energy surfaces (FES) at b 200 GPa, c 300 GPa, d 400 GPa and e 600 GPa, all computed at 1000 K, as a function of two collective variables: CV1 measures the anistropy of the box and CV2 measures the tilt of the box. Representative snapshots of the ice structures collected during the metadynamics simulations are shown. SF1 and SF2 are structures that are similar to Pbcm but with different stacking sequences. The green diamond symbols indicate the location of X(cubic) on the FES.
