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

[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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>