[Wien] regarding exciton binding energies and dielectric constant [updated]
Dr. K. C. Bhamu
kcbhamu85 at gmail.com
Thu Nov 10 18:26:43 CET 2016
ohhh, sorry Prof. Peter.
(sp) was included by mistake. I forgot to delete it.
For Pb(5p): When I initialized the case I for one 5P state as core(T) and
other as valance (F).
The total charge leakage (not in the initialization bu it was observed in
the *.outputst file) was 0.00068 with -6.0 ecut and with -7.5 it was
0.000126. So I thought it would be better it I use -7.5 ecut.
However, I will now use -ecut as -6.0.
I think now the step I am going to follow are fine??
One query was unanswered:
how iqtlsave will change the calculation if I choose it as "0"?
Bhamu
On Thu, Nov 10, 2016 at 10:35 PM, Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:
> Why would you do a runsp_lapw for a non-magnetic system ???
>
> Spin-orbit is also active for a non-magnetic material.
>
> Don't mix up spin-polarization with spin-orbit ....
>
> PS: I'm also not sure why you want to include the Pb-5p state as valence
> ?? I don't think you have to use so small Pb spheres that you get core
> leakage ?? And with Pb-5p as core you have the full SO-splitting inluded,
> ....
>
>
>
> 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 <mailto: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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>> --
>> ------------------------------------------------------------
>> --------------
>> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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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
> Email: blaha at theochem.tuwien.ac.at WIEN2k: http://www.wien2k.at
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