[Wien] regarding exciton binding energies and dielectric constant [updated]
Peter Blaha
pblaha at theochem.tuwien.ac.at
Thu Nov 10 18:05:57 CET 2016
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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