# Dielectric susceptibility

Hi everybody,

i'm trying to calculate the dielectric susceptibility of Silicon (Si) using the formula

$$\chi_{\vec g}=S(\vec g)(-\frac{\omega_p^2}{\omega^2})\frac{F(\vec g)}{Z}e^{-M}$$

where $S(\vec g)$ is the structure factor, $\omega_p$ is the plasma frequency, $\omega$ is the frequency, $F(\vec g)$ is the atomic form factor, Z is the atomic number of Silicon (14) and M is the Debye-Waller factor (i take it equal to zero).

I'm interested in (111) planes, so $S(\vec g)=4+4i$.

The problem is that i'm not getting the correct result in my calculations. I'd like to know if i can find somewhere values of the dielectric susceptibility with respect to the energy $\omega$. I work with physical units. Thus $\omega$ is given in eV and i'm in the X-ray region (keV).

Additionally i'd like to know if the values from http://physics.nist.gov/PhysRefData/FFast/html/form.html" [Broken] that i use for the atomic form factor $F(\vec g)$ are correct or whether i need to make some changes.

What confuses me is that only the outer 2 electrons contribute to the flux in the crystal and since the crystal system of Si is fcc with 2 atoms base that means than in every unit cell exist $(8\frac{1}{8}+6\frac{1}{2})\times 2 \times 2$ free electrons. Do i need to include that somewhere in the formula above?