[Pw_forum] Regarding Dielectric constant calculation

[bramha pandey]

Dear QE users and developers...
I was trying to calculate the static dielectric constant at gamma point. My
system is AlN (ZB, fcc, 2 atom per unit cell). PP's are Norm-conversing for
Al and N both.
As i optimize the system wrt ecut and kpoint mess with 40 Ry and 4*4*4 and
at this value by phonon program i obtained dielectric tensor=5.67 approx
which is much less than experimental (9.0).
But when i take the ecut=18Ry and kpoint=4*4*4, i was able to find the near
value of dielectric tensor as 8.57 approx and also the theromodynamic
property Cv is coming approx near about experimental value except
some variation at lower temperature(up to 100K).

My question is that should we  perform the phonon calcualtion in
unoptimized case i.e  we should always put ecut as min for correct
calculation of phonon property. At this point i am very much confused.

 Please any type of comment is highly appreciable.

-- 
Thanks and Regards
Bramha Prasad Pandey
GLA University, Mathura.
INDIA.
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