comparison with existing experimental spectra as well. We discuss the electronic excitation structure and the optical spectra including the existence of bound excitons, and compare with available experiments.[@kalugin] One of our goals is to provide a wide scenario of the one- and two-particle properties of this material treated within first-principles techniques. However, as considered later on, a very extended high-energy tail and overall rather small values of the dielectric function require inclusion of an unusually large number of bands or band pairs in the GW and BSE schemes. In regard to this last point, we suggest a theoretical framework and a simulative protocol able to give solutions with appreciable agreements with the available experimental measures, saving at the same time the computational demands.

Ground state properties and computational details {#sec:Ground state properties}
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All the calculations have been performed using density functional theory (DFT)[@kohn] as implemented in the plane-wave basis code VASP.[@kresse; @kresse2] The projector augmented wave (PAW)[@paw; @paw2] method is applied to generate pseudopotentials and wave functions in the spheres around the cores. In standard use, VASP performs a fully relativistic calculation for the core-electrons and treats valence electrons in a scalar relativistic approximation.[@kresse; @kresse2; @bachelet; @kaupp] The effect 