Many-body Green's function calculations of optical properties of LiF in extreme conditions

We present Density Functional Theory (DFT) + quasiparticle ($G_0W_0$) + Bethe-Salpeter calculations of the real and imaginary parts of the long-wavelength dielectric function of LiF between ambient pressure and $P$= 5 Mbars. While the optical absorption spectrum is predicted to show dramatic pressure-dependent features above the optical gap, the index of refraction well below the gap is shown to exhibit the same trends as that seen in both DFT calculations and experiment: a linear increase with $P$. This increase does not result from a decrease in the band gap, but rather follows from the increase in oscillator strength which counteracts a smaller increase in band gap with $P$. Our calculations also suggest that the index of refraction (for visible and near-UV light) of the higher-$T$ B2-phase should be close enough to that of the B1 (ambient crystalline) phase that a transition from B1 to B2 is not likely to present a substantial change in index. These findings may be of interest to researchers who use LiF as a window material in dynamic compression experiments.