Hi,
as far as I understand SIESTA can compute the dielectric function directly.
But for phenomenological analysis you don't really need SIESTA itself.
I believe you meant flat-band analysis (based on the amount of search
results in Google I suppose "fat-band" is just a mistype found in
several papers which spreads to more and more other papers).
All you need is the electronic band structure plot.
As in DOS the peaks come from the regions where bands are flat or have
an extremum, similarly in dielectric function, which is basically the
joint DOS (i.e. density of pairs of states with given energy difference)
modified by the oscillator strength (which further improve the
transitions between parallel bands), the peaks would come from regions
where bands are parallel.
To simplify this analysis you may plot the joint band structure (or
energy band differences) i.e. instead of band1, band2, band3 as in band
structure, plot the band3-band2, band3-band1, etc. and look for flat
bands and extremums.
See for example http://prola.aps.org/pdf/PRB/v51/i19/p13516_1
You can modify the SIESTA band structure output, but even simpler would
be to write a small script or program that would take the SIESTA
electronic structure output and rewrite the band differences into a new
file.
Good luck.
Alexander.
