[Wien] regarding exciton binding energies and dielectric constant [updated]
Dr. K. C. Bhamu
kcbhamu85 at gmail.com
Thu Nov 10 17:24:54 CET 2016
Thank you Professor Peter and McLeod for your nice explanation.
I have to go through the paper and then I can make a better statement.
You are right Mcleod, it is orthrohombic MAPbI3 structure.
Dear Prof. Peter,
It is reported by Tran that mBJ is not good for effective masses:
http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg10700.html
However as per your suggestion I am going to follow below steps. I have
limited wall time so I need your comment on below steps:
For now (to save walltime), Please make any comment in any step if I am
going wrong:
What I am using:
*Goal*: mBJ+SO
A.
1. PBE as PBE gave better band gap so it may be useful. In the suggested
paper they did not used PBE function instead they used PBEsol.
2. 144 k-points generated, ndiv= 11 7 11 (I think it is more than
sufficient).
3. ecut -7.5 to consider Pb (5P) state as valence.
4. R-MT*K-MAX = 3 (as I have "H" here).
5. gmin = 9.23077, gmax = 20.00000
6. SP calculation ? No (as it is not a magnetic system).
B.
run a regular scf "min -j 'run_lapw -p -i 40 -ec 0.0001 -cc 0.0001 -fc 1'
Tt will give me a relaxed structure with optimised a,b and c.
C.
1. save_lapw case_bhamu
2. init_mbj_lapw
3. run_lapw -i 1
4. run_mbj_lapw
"run-lapw -p -i 80 -ec 0.0001 -cc 0.001" (as per your recent
comment on my post that we do not
need -fc if we have done in regular scf)
5. save_lapw case_mBJ
D.
1. initso_lapw
2. runsp_lapw -so -p -i 40 -ec 0.0001 -cc 0.0001' Fine ????
I am using both step C and D differently because Dr. Tran suggested for the
same (
http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg03843.html)
One more question:
how iqtlsave will change the calculation if I coose it as "0"?
Kind regards
------------------------------------------------
Dr. K. C. Bhamu
(UGC-Dr. D. S. Kothari Postdoc Fellow)
Department of Physics
Goa University, Goa-403 206
India
Mob. No. +91-9975238952
On Thu, Nov 10, 2016 at 8:00 PM, Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:
> 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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>>>>
>>>
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>
> --
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
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