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

[Peter Blaha]

Very good explanation.

So you should probably use SO + mBJ and see what comes out then ....
(you should get again a good band gap, although effective masses are not 
necessarily improved by mBJ ...)

Am 10.11.2016 um 15:24 schrieb John McLeod:
> I have some experience using WIEN2k for metal organic halide perovskites.
>
> PBE without SOC gets the correct band gap for CH3NH3PbI3 (which I assume
> is the compound Dr. Bhamu is studying) because of a "fortuitous" error
> cancellation between using PBE and ignoring SOC. This is reasonably well
> known and has been studied in detail in several manuscripts. SOC+PBE
> results in a significantly underestimated band gap, as one might expect.
>
> I assume Dr. Bhamu is using the calculated low frequency dielectric
> constant (e*), and the calculated effective mass (m*) to estimate the
> binding energy using the simple Mott-Wannier model: E_ex = m*/e^2 (13.6)
> eV .
>
> SOC does modify the shape of the bands near the gamma-point (I believe
> it reduces the effective mass), and SOC also influences the dielectric
> constant. So I think perhaps including SOC and using a scissors
> operation with OPTIC to get the correct band gap may be the most
> straight-forward (if not completely ab initio) method.
>
> Have you looked at F. Brivio, et al., Phys. Rev. B 89 155204 (DOI:
> 10.1103/PhysRevB.89.155204)?
> They go into some detail about different approaches, it may be helpful
> for your present situation.
>
> Regards,
> -John McLeod
>
> So I do not think SOC can be
> On 2016-11-10 10:02 PM, Peter Blaha wrote:
>> 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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>>> Wien at zeus.theochem.tuwien.ac.at
>>> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
>>> SEARCH the MAILING-LIST at:
>>> http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
>>>
>>
>
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