[Wien] regarding dielectric constant

[Pavel Ondračka]

Dr. K. C. Bhamu píše v Út 15. 11. 2016 v 18:16 +0530:
> Dear Prof. Peter 
> 
> This is in continuation with my previous query for excitation binding
> energy and dielectric constant.
> The first line of the case.epsilon is:
> 
> # Energy [eV] Re_eps_xx     Im_eps_xx     Re_eps_zz     Im_eps_zz
> #
>    0.013610  0.556574E+01  0.571125E-01  0.557539E+01  0.591910E-01
> 
> According to this data, the static dielectric constant comes out to
> be: ~5.56. What does it mean? Should it be considered for low
> frequency dielectric constant?
> In our measured value we are getting low frequency dielectric
> constant ~18 which is quite larger than calculated. We tried to
> calculate low frequency dielectric constant using plazma frequency
> and we got it 17.8 which is nearly equal to the measured value. For
> this we used average band-gap value 
> 
> 
> In the literature I am getting low frequency dielectric constant
> value between epslion_0 17-24 and at epscilon_infinite ~4-6 for
> CH3NH3PbI3.
> 
> I need any comments on it.

Dear Dr. K. C. Bhamu,
to repeat what was said before, this is expected. What you get from the
optic calculation (the case.epsilon) file is only the electronic part
(and not even the whole electronic part) of the dielectric constant.
This probably corresponds to the epsilon_infinity you are mentioning
(but its hard to tell since epsilon_infinity usually stands for
contributions to dielectric function from some higher energy processes
but it does not say anything about what processes these are).

As prof. Blaha said, to get the ionic part of the static dielectric
constant you would need to do the BERRYPI calculations to get Born
effective charges tensors. From my understanding (but I'm in no way an
expert here, so hopefully someone can correct me if I mess something
up) I think you would also need to do the phonon calculations and then
you can combine it with the Born effective charges to get the mode-
oscilator strengths and finally the ionic part of the static dielectric
tensor, see eg. this article (first three equations) for some info:
Rignanese, G.-M., Gonze, X. & Pasquarello, A. First-principles study of
structural, electronic, dynamical, and dielectric properties of zircon.
Phys. Rev. B 63, 104305 (2001).

BTW if someone here at the mailing list did already calculated the
ionic part of the static dielectric constant, the mode-oscilator
strengths or anything related to IR optical absorption with Wien2k I
would be interested to hear about it.
> 
> Now using the average band-gap, we calculated " excitation binding
> energy" which is now ~8 far below the experimental value 13.
> This difference in the excitation binding energy may be due to no-
> inclusion of the SO effect (or mBJ+SO as per your suggestion in last
> email).

The wise move would be to add the SO as was already advised by John
McLeod and then deal with the band gap problem (scissor operator, mBJ
or +U) to see it this improves your results.

Best regards
Pavel