Hydrogen-bond potential for ice VIII-X phase transition

Repulsive force between the O-H bonding electrons and the O:H nonbonding pair within hydrogen bond (O-H:O) is an often overlooked interaction which dictates the extraordinary recoverability and sensitivity of water and ice. Here, we present a potential model for this hidden force opposing ice compression of ice VIII-X phase transition based on the density functional theory (DFT) and neutron scattering observations. We consider the H-O bond covalent force, the O:H nonbond dispersion force, and the hidden force to approach equilibrium under compression. Due to the charge polarization within the O:H-O bond, the curvatures of the H-O bond and the O:H nonbond potentials show opposite sign before transition, resulting in the asymmetric relaxation of H-O and O:H (O:H contraction and H-O elongation) and the H+ proton centralization towards phase X. When cross the VIII-X phase boundary, both H-O and O:H contract slightly. The potential model reproduces the VIII-X phase transition as observed in experiment. Development of the potential model may provide a choice for further calculations of water anomalies.