[Wien] metal supercell band folding dielectric function

[MingWenmei]

Dear all,
I am calculating the interband dielectric function of Au with low concentration of Cd dopant (say 1/32 atom concentration). Very strikingly in low energy (< 1eV) range, there appears a high peak (~a few hundred). However for FCC pure gold, the interband dielectric function is very small (around zero) below 1eV. 
So my question is physically why the dielectric function changes this dramatically? I did the band structure calculation. I found within small energy range (<1eV) of Fermi energy for the doped system, there are some extra bands. I began to realize it is this small energy difference of the bands above Fermi energy that causes the high peak of dielectric function. 
These extra bands may be from doped Cd and also may be from the band-folding of host Au in the supercell calculation. The contribution from supercell itself to the dielectric peak seems not to make sense physically. For if band-folding can affect the value of dielectric function, then even for pure FCC Au we may get significantly different dielectric functions by choosing "unit cell" of different size (say 1x1x1, 4x4x4 times primary cell )
Could anyone give some comment to understand this? 
Thank you a lot,
Wenmei



 		 	   		  
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