Figure 2.

Illustration of the problems in choosing the reliable parameters for $Re{\epsilon }^{free}\left(\omega \right)$ in the low frequency range using the parameters of the imaginary part well fitted to low-frequency $Im{\epsilon }^{exp}\left(\omega \right)$ dependence (a, blue dashed lines). The additives to ${\epsilon }^{free}\left(\hslash \omega \right)$ (red lines) which describe frequency dependent interband transitions (Section 3.3) are presented in insets for gold and silver (a,b, respectively). The resulting ${\epsilon }_m\left(\omega \right)$, Equation (3), reproduces well ${\epsilon }^{exp}\left(\omega \right)$ and $n\left(\omega \right)$ in a broad spectral range (see blue lines in Figure 1).