Figure 1: Refractive indices.

Pressure dependence of the refractive index, n, of water and ice. We computed , where ₁ is the real part of the electronic dielectric constant, obtained from ab initio simulations (see text). The experimental data for ice VII, obtained with light wavelength of 630 nm, are from ref. 16. The experimental values for water at 673 K are from ref. 22. We note that the statistical fluctuations in the value of n are rather small, indicating that ₁ is largely insensitive to reorientations of the molecules in the fluid and to hydrogen bonding rearrangements, unlike the ionic dielectric constant²⁶. When increasing the temperature from 1,000 to 2,000 K, n showed again moderate variations. The error bars show the s.d. values of the refractive indices of water. Lines are drawn to guide the eyes only. Calculations were conducted for ice VIII instead of VII for computational simplicity, as ice VII is proton disordered.