[Pw_forum] XSpectra - tail of the spectrum falling relatively quickly

[Cristian Degli Esposti Boschi]

Dear Yevgen,

I would like to contribute my little experience with XSpectra to answer
your question about the difference between computed and experimental
spectra at high energies.
In fact it is a problem we have encountered
in all the cases we have studied with a student of mine (Emiliano
Burresi), both for test systems and for nontrivial clusters of
metalloproteins.

I made the same questions about four years ago to Matteo Calandra,
the main developer of the XSpectra module, and I translate below
the essential part of the answer...

"At high energies (in the EXAFS region) the theoretical curve tends to
zero while the experimental one doesn't.
The reason lies in the way the projectors of the PAW method are built in
Quantum Espresso; they have no nodes and as such they are no longer a
valid basis at high energies. The decay to zero of the theoretical curve
is an artifact, actually it should settle to a constant.
For this reason XSpectra allows for a calculation of XANES (X-ray
absorption NEAR EDGE structure) but not EXAFS."

With Emiliano we simply considered a test case (Silicon) and tried
to increase the number of projectors in the PAW scheme; as shown in the
figure here (not published)

https://dl.dropboxusercontent.com/u/47323550/XANES_Si.tif

the spectrum goes at least in the right
direction, so this may be a viable solution even if not particularly
efficient.

Now, another question is if the situation has changed since 2010,
and I cannot be more precise on this.

Hope this helps. Cristian

-- 
Cristian Degli Esposti Boschi
CNR-IMM, Sezione di Bologna,
via Gobetti, 101, 40129, Bologna, Italia
tel. ++39 051 6399152, fax ++39 051 6399216
email: degliesposti -AT- bo.imm.cnr.it
web:   www.bo.imm.cnr.it/site/staff/personal_pages/degliesposti/