derived from this calculation. Thus, an important methodological point is the following: From each optical function produced here one can return to the original dielectric function which was the output of the code we used (”our reference result”). Moreover the comparison with the experimental result for $Im ~\varepsilon (\omega)$ is anyway excellent (see Fig. \[fig:3\]). If one operates a broadening to fit at best an optical function with respect to a particular experiment, the reverse operation (to generate again the $Im ~\varepsilon (\omega)$ in agreement with experiment) is not guaranteed.

In Fig. \[fig:7\], we present the energy loss function after the present BSE calculation and experiment in a wide energy range.[@kalugin] Also for the onset region and the main three structures below 20 eV the agreement is very close in positions and, a little less, in the oscillator strengths. For energies larger than 20 eV, a slight blue-shift of the peak positions is obtained with respect to the experimental ones. Furthermore, some differences with respect to the energy loss function $-Im ~\varepsilon^{-1}$ of other semiconductor/insulator systems could be considered. Indeed, these systems generally show a broad single structure around the plasmon frequency $\omega_p$ (e.g., as in the cases of cubic 