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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