[Wien] regarding exciton binding energies and dielectric constant [updated]

[Peter Blaha]

I'm not the expert on that topic, but I think you mix up the two 
dielectric constants, which could be a semantic problem. To compare with 
a classic experiment, you may need to obtain the ionic contribution to 
the dielectric constant, which as far as I know can be done using BERRYPI.

Other comments:
To obtain the "correct" band gap using PBE is very "unusual". For most 
materials (but of course there could be exemptions) the PBE band gaps 
should be ~50%  smaller than experiment.

Pb ??? this is very "relativistic" ! Did you consider spin-orbit coupling ?

And last but not least, I have no idea how you calculate exciton binding 
energies from a single particle spectrum. We would do this using BSE 
calculations, but your system is probably too complicated for this.

Am 10.11.2016 um 14:26 schrieb Dr. K. C. Bhamu:
> Dear Prof. Peter and Experts
> This is with some more information:
>
> To put a joint paper on complex Metal-organic halide perovskites, I am
> trying to reproduce some experimental results measured by my collaborator.
>
> For my complex system, I got low frequency dielectric constant value of
> ~5.6 (at 0.013 eV) and the calculated the exciton binding energy  ~0.087
> - 0.095 eV  (85 -97 meV). This is too high because the measurements here
> get about 13 meV and a 1-2 transition of ~9.9 meV (measured).
>
> In literature the reported static and optical dielectric constants for
> the system are in the range of 17-24 and 4.5-6.5 respectively using DFT.
>
> In my case the zero frequency dielectric constant (~ 5.6) is in tune
> with the optical dielectric constants (4.5-6.5).
>
> I think my value ~5.6 should be in the range of 17-24. *Is it so?*
> Please help me to understand it.
>
> I used PBE functional with 4x4x4 k mesh. I reduced rmt by 5% and then
> rmt for Pb and I were reduced by a factor of 0.3. I have doubt here??
>
>  My band gap is in reasonable agreement with the experimentally observed
> band gap (1.57eV) +/- 0.1.
>
> The problem may be that my epsilon value (~5.6) is too low and I looked
> up our local measured value of ~18 for the low frequency part. If I use
> this value (18) then much better exciton binding energies come out.
>
> What can be an mistake that I may did in calculation? or may it be a
> reason of the device fabrication because for experimental part some
> p-i-n and n-i-p type device has been framed?
>
>
> Kind regards
>
> Bhamu
>
>
>
>
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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