2040 eV.

The reflectivity function $R$ is show in Fig. \[fig:6\] in comparison with an experimental spectrum[@bourdillon]. The positions of the main peaks of the theoretical curve (namely, occurring at 7.8 eV, 13.2 eV, and 15.2 eV) reproduce the corresponding ones of the measurement, except for the fourth peak (at 18.0 eV) which is overestimated in intensity and it should be compared with the shoulder around 17 eV as obtained in the experimental spectrum. Furthermore, the energy region above 16 eV is only in qualitative agreement with experiment. Regarding the first three peak positions of the reflectivity *R*, our results compare well with the experimental values after Berger and coworkers.[@berger] Albert *et al.*[@albert] assign these peaks to $\Gamma_{15}^{V}\rightarrow \Gamma_{1}^{C}$, $X_{2'}^{V}\rightarrow X_{3}^{C}$ and $X_{5'}^{V}\rightarrow X_{3}^{C}$ excitons. The first excitonic peak at 7.7 eV is consistent with the results of 7.6 eV given by Orlowski[@orlowski], Raisin[@raisin], and by Forman[@forman] and coworkers.

We focus on the fact that in our computations a natural/instrumental broadening of 0.2 eV has been applied in all the curves. Within our scheme, ”our experimental curve” is the imaginary part of the dielectric function, $Im ~\varepsilon (\omega)$ (the first outcome of the computational code). All the other functions are 