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

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