Proton dynamics in high-pressure ice-VII from density functional theory

Recent Nuclear Magnetic Resonance (NMR) data on high-pressure ice-VII (Meier et al., Nature Comm., 2018) have revealed significant mobility of protons in the pressure range of 20-95 GPa at room temperature. Using a density-functional-theory-based approach, we explore the compression-dependent proton dynamics in ice-VII by directly sampling the proton potential along the diagonal O-O direction in the disordered body centered cubic configuration. We describe a configuration showing a double-well potential with a barrier that permits tunneling at compressions corresponding to 20 GPa. The double-well character disappears near 45 GPa, but a broad minimum indicates significant proton mobility to persist to 95 GPa. Tunneling frequencies are computed using the approximation of Wentzel, Kramers and Brillouin, with frequencies in the THz range.