Ab initio simulations of dielectric and optical properties of ices I$_{\mathrm{h}}$, I$_{\mathrm{II}}$ and lattices of hydrates sI, sH

The results of calculating the dielectric and optical characteristics of solid polymorphic phases of water ices I$_{\mathrm{h}}$, I$_{\mathrm{II}}$ and lattices of hydrates sI, sH are presented. Static dielectric tensors $\varepsilon_{ik}$ and complex frequency-dependent tensors $\varepsilon_{ik}(\omega)$ are calculated for these materials. It is shown that, in terms of optical properties, the crystal lattices I$_{\mathrm{h}}$, I$_{\mathrm{II}}$, and sH are uniaxial, the tensor components $\varepsilon_{xx}(\omega)$ and $\varepsilon_{yy}(\omega)$ coincide for them, and the hydrate lattice sI is isotropic. Based on the calculated frequency-dependent dielectric functions $\varepsilon'_{ik}(\omega)$ and $\varepsilon''_{ik}(\omega)$, important optical characteristics were obtained: reflection $R(\omega)$, absorption $a(\omega)$, loss function $L(\omega)$, refractive indices $n(\omega)$ and $k(\omega)$. Comparison of the dielectric and optical spectra of the sI and sH gratings with the known spectra for methane hydrate sI revealed a broadening of the spectra in the high-energy direction. For the unfilled hydrate sI, a reflection peak was found at an energy of 17.3 eV, the appearance of which is associated with a change in the electronic structure of the crystal in the absence of a methane molecule. Qualitative agreement is obtained between the reflection spectra $R(\omega)$ and the functions $\varepsilon'_{ik}(\omega)$, $\varepsilon''_{ik}(\omega)$, calculated by quantum mechanical simulation, with experimental spectroscopy data for hexagonal and amorphous ices.