It was recalled that the shape of the proton-transfer potential between a donor and an acceptor heavy atom depended greatly upon the length of the H bond. It changed from a double-well, to a single-well, shape in the limit of very short H bonds. Therefore, highly compressed ice was an ideal material in which to study systematically the effect of this change upon the properties of H-bonded solids. A computer simulation technique was used that treated the nuclei as quantum particles and also included electrons within the framework of density functional theory. On the basis of this first-principles approach, 4 different regimes were identified. In the limit of long/weak H bonds, the proton was covalently bound to a particular heavy atom, and zero-point motion broadened its distribution. Ionic defects played only a minor role. In the case of shorter bonds, a regime was found with strong proton tunnelling between the donor and acceptor. This phase was characterized by a high concentration of ionic defects. At yet shorter distances, the proton resided mid-way between donor and acceptor, due to zero-point motion, in spite of the double-well nature of the underlying potential. Very short (ultra-strong) H bonds were symmetrical because the potential profile was of single-well type.

The Role of Quantum Effects and Ionic Defects in High-Density Ice. M.Benoit, D.Marx, M.Parrinello: Solid State Ionics, 1999, 125[1-4], 23-9