Nature Commun. 5, 3919 (2014)

Credit: © 2014 NPG

The properties of water and ice under a wide range of temperatures and pressures are well established. However, it is challenging to determine their electronic and dielectric properties under extreme conditions, because of the experimental difficulties of performing the required optical measurements. Now, ab initio molecular dynamics simulations of water under pressure performed by Ding Pan and co-workers at the Universities of California and Chicago have enabled the refractive index and electronic gap of water and ice to be precisely calculated for pressures up to 30 GPa. The results reveal that both parameters increase with increasing pressure. This goes against the widely held view that the bandgap of water, like that of many other materials, is inversely correlated to the measured refractive index. It also turns out that the single-oscillator model, which is extensively used to determine the electronic properties of various materials, is insufficient for modelling the properties of water under extreme pressure and hence needs to be revisited.