[Wien] Bethe-Salpeter Equition

[Kevin Jorissen]

Dear Chikashi,

from your question I guess that you do not know the theory behind BSE and
WIEN2k, so I will give some background information:

Bethe-Salpeter is a whole different game: it is a completely different
theory than Density Functional Theory (DFT).  BSE solves an explicit
many-body equation (the basis consists of two-particle wave functions)
while WIEN2k/DFT solves an effective single-particle equation (Kohn-Sham
equation).  WIEN2k does not include BSE calculations.

The codes that do BSE are generally harder to install and to use than
WIEN2k.  They also take much longer to calculate (you need more CPUs and
more memory).

There is a separate code based on WIEN2k that does BSE calculations.  There
are also other codes such as Exc!ting, OCEAN, and a few others that can do
the BSE calculation.   Again, these calculations are generally a bit more
difficult than WIEN2k.

The strength of the core hole (0.6 vs. 1.0) is an arbitrary parameter.
 Roughly speaking it represents the strength of the screening (i.e. how
quickly and efficiently surrounding electrons respond to the presence of
the core hole and "screen" it, thus modifying the interaction between the
core hole and the photoelectron resulting from the excitation).  This
approach is an approximation to the true many-body description of the
problem (such as BSE) and therefore it is justifiable to use whatever value
of the core hole gives the best results.  In most cases people use 0, 0.5,
or 1.0 because these can be conceptually understood.  But as I said,
whichever value gives the best result compared to experiment, is obviously
the best "static" description of the screening that you will be able to
find in a DFT calculation, so go ahead and use 0.6.  In the BSE you would
not have this parameter of core hole strength per se.  The screening will
be calculated (and is frequency dependent, not just a number).  However
there may be other somewhat ad hoc parameters in a BSE calculation, and
such calculations are less established routine than a DFT calculation.

If you get a good result from using WIEN2k with a 0.6 core hole, I'd
personally declare victory and stop there.  Just add 2 sentences about
screening to your paper and referees will make no problem of that.
 Apparently your material is sufficiently well described by a DFT
Final-State-Rule approach.


Also, you should go to wien2k.at and download a newer version of WIEN2k.
 Your version is 4 years old and has many bugs that have since been fixed,
as well as improved performance and new functionality.

Cheers,

Kevin




On Wed, Sep 3, 2014 at 9:51 PM, "Chikashi Suzuki (鈴木知史)" <
suzuki.chikashi at jaea.go.jp> wrote:

> Dear Prof. Blaha and Wien2k users
>
> My name is Chikashi Suzuki, from Japan Atomic Energy Agency.
> I am running wien version WIEN2k 10.1.
>
> I have tried to caclulate Cs M4,5 Xay absoprtion spectra of Cs compound.
> Cs M4,5 XANES of CsCl is well reproduced
> in case where the core hole is 0.6 (not 1.0),
> though I cannot find this reason.
>
> I found some studies where XANES spectra are well-repsoduced
> using Bethe-Salpeter Equition with Wien2k code.
> However, I cannot find how to calculate  XANES spectra using
> Bethe-Salpeter Equition with Wien2k code.
> (For example, what input file to use, and what to write
>  in the input file)
>
> What should I do in order to calculate XANES spectra using
> Bethe-Salpeter Equition with Wien2k code.
>
> Sincerely yours
>
> ***********************************
> Dr. Chikashi Suzuki
> Research Group for Accident Progression Analysis
> Nuclear Science and Engineering Center
> Japan Atomic Energy Agency
> Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan
> Tel: +81-29-282-5386
> E-mail：suzuki.chikashi at jaea.go.jp
> ***********************************
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