[Wien] Ghost band error in SnS2 continued....
Peter Blaha
pblaha at theochem.tuwien.ac.at
Tue Jan 28 20:14:59 CET 2020
Just to explain the mechanism how a U for "semicore" 3d states (Sn, but
also Zn in ZnO or Ge, ...):
Usually, U shifts occupied d-states down by U/2 and unoccupied d-states
up by U/2. This is the mechanism eg. in NiO.
However, for these fully occupied states, the effect of U is much
smaller, since ther is no "reoccupation" possible.
Now: how can the gap, which is formed between O-2p states at the VBM and
Sn-s states at the CBM be affected by a U for d states ??
The effect is very "indirect": The U localizes the d-semicore states
even more, this leads to a better shielding of the nucleus as seen by
the very diffuse 4s (for Zn) states, i.e. this means that these s states
are less attracted by the nucleus and increase their energy. Thus the
larger gap.
It is clearly not the correct physics, as even an exact DFT functional
should not give you the proper gap as difference of VBM and CBM (without
the xc-discontinuity), but might mimic it well if the U is chosen
properly (and very large).
Am 28.01.2020 um 17:11 schrieb Laurence Marks:
> An important point Peter pointed out to me some years ago.
>
> Remember that electrons are dumb. They go where they want; we think
> about them as s,p,d etc. I suspect that your Sn d-states should be
> semi-core (-1.5Ryd) and largely uneffected by +U
>
> What you (and perhaps your cited paper) are probably really doing is
> changing the tails of the S states that reach into the Sn RMT. These
> have the symmetry of d-states, so are counted as such and "see" the
> Hubbard term. This is not really correct physics, more a fudge to avoid
> have too much covalency.
>
> On Tue, Jan 28, 2020 at 8:36 AM shamik chakrabarti
> <shamik15041981 at gmail.com <mailto:shamik15041981 at gmail.com>> wrote:
>
> I want to simulate Li intercalation voltage in SnS2. But for that I
> need to simulate pristine SnS2 properly....
>
> On Tue, Jan 28, 2020, 19:46 Laurence Marks <laurence.marks at gmail.com
> <mailto:laurence.marks at gmail.com>> wrote:
>
> Reducing the RMT by 30% is somewhat large.
>
> The key question is what are you trying to do? Reproducing a
> result with Vasp is not good science if that is your purpose.
> Without that information I doubt that anyone can provide useful
> advice
>
> _____
> Professor Laurence Marks
> "Research is to see what everybody else has seen, and to think
> what nobody else has thought", Albert Szent-Gyorgi
> www.numis.northwestern.edu <http://www.numis.northwestern.edu>
>
> On Tue, Jan 28, 2020, 08:11 shamik chakrabarti
> <shamik15041981 at gmail.com <mailto:shamik15041981 at gmail.com>> wrote:
>
>
> Dear Professor Laurence and wien2k users,
>
> with reference to my
> earlier mail on SnS2, I have checked the change in RMT (for
> avoiding Ghostband) with U and corresponding band gap. The
> details are as follows;
>
> U = 7 eV RMT reduction 6% Band gap = 1.460 eV
> U = 8 eV RMT reduction 15% Band gap= 1.475 eV
> U= 9 eV RMT reduction 30% Band gap = 1.512 eV
>
> I have not able to reproduce the band gap ~ 2.1 eV as
> computed earlier using Vasp (Phys. Chem. Chem. Phys., 2016,
> 18, 318). Also, whether 30% reduction of RMT is acceptable???
>
> Looking forward to further guidance from you.
>
> with regards,
>
> --
> Dr. Shamik Chakrabarti
> Research Fellow
> Department of Physics
> Indian Institute of Technology Patna
> Bihta-801103
> Patna
> Bihar, India
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> --
> Professor Laurence Marks
> Department of Materials Science and Engineering
> Northwestern University
> www.numis.northwestern.edu <http://www.numis.northwestern.edu/>
> Corrosion in 4D: www.numis.northwestern.edu/MURI
> <http://www.numis.northwestern.edu/MURI>
> Co-Editor, Acta Cryst A
> "Research is to see what everybody else has seen, and to think what
> nobody else has thought"
> Albert Szent-Gyorgi
>
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--
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