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

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

where [itex]S(\vec g)[/itex] is the structure factor, [itex]\omega_p[/itex] is the plasma frequency, [itex]\omega[/itex] is the frequency, [itex]F(\vec g)[/itex] 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 [itex]S(\vec g)=4+4i[/itex].

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 [itex]\omega[/itex]. I work with physical units. Thus [itex]\omega[/itex] 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 [itex]F(\vec g)[/itex] 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 [itex](8\frac{1}{8}+6\frac{1}{2})\times 2 \times 2[/itex] free electrons. Do i need to include that somewhere in the formula above?

Thank you all in advance,

John

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# Dielectric susceptibility

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