[Pw_forum] x-ray spectra with "Fermi-Dirac" electronic smearing

jydai daijiayu at nudt.edu.cn
Thu Apr 29 11:39:34 CEST 2010

Dear Matteo,

Thanks for your detailed answers. Yes, i want to consider the electronic contribution to the K-edge XAS when temperature is high, .ie., above 1 eV. This can be reached by adding the Fermi functions for the electronic distribution, i think. 

However, people are doing on this project without this contribution now, as said in the papers i sent to you before. So, i think it also can be solved partly by the dipolar or more detailed considerations as you said. 

I will try again as your suggestions, thanks a lot. 


   if I understand correctly you would like to calculate K-edge XAS at 
high temperature including the contribution of Fermi functions in the
Fermi golden rule.
If this is what you would like to do, this is currently not implemented 
in XSPECTRA, as we always assume the temperature to be negligible with 
respect to the energy of the electronic excitations (1 eV=11605 Kelvin).

In practice we assume that the XAS spectra is temperature independent,
or, better, the only temperature dependence comes from the thermal
expansion of the crystal structure.
This is generally true except when vibrations contribute to the XAS
spectra. Increasing temperature increase atomic vibrations and
it is known in some case to wakly affect the spectra in the pre-edge 
However even in this case I guess that the problem can be solved
without the need of introducing Fermi functions in the XAS
cross section.

You will find more details on the XAS equation implemented in
Christos Gougoussis, Matteo Calandra, Ari P. Seitsonen, and Francesco 
Mauri, Phys. Rev. B 80, 075102 (2009)
Equation 1.

What the fermi_level option in XSPECTRA does is simply
calculating the Fermi level with a given electronic smearing. The 
electronic smearing is used in the case of metals to improve the 
sampling of the Fermi surface when a finite grid of k-points is used.
In the case of a metallic system, as your case is, then XSPECTRA
does not calculate the Fermi level and stops as this quantity
can be simply read from the pw.x scf output that you must have run 
before XSPECTRA.
Finally, note that to calculate the electronic structure and the
cross section, any smearing available in pw.x should also
work on XSPECTRA.
However this is not equivalent in adding Fermi functions to the
Fermi golden rule.

Jiayu Dai
Department of Physics
National University of Defense Technology, 
Changsha, 410073, P R China

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