[Pw_forum] XSpectrum

[Stefano Baroni]

Because in that case the code would assume that the excited electron occupies the lowest available conduction state, whereas in the experiment that you are correctly describing the core electron would be excited to some high-lying conduction band. On more physical grounds, the code simulates photoionization process, rather that than a photoexcitation one. When the final energy of the excited electron is high enough, however, the two processes are indistinguishable. SB

On Dec 8, 2011, at 9:46 PM, Hanghui Chen wrote:

> To QE developers,
>      In the examples of X-ray Spectrum calculations, I noticed that for diamond, there are two different calculations. In one of them (the so-called Z approximation), 8 normal carbon atoms are employed. In the other one (Z+1 approximation), in addition to 7 normal carbon atoms, a special carbon atom is employed with a fixed hole in its valence (basically, it is N atom. Correct me if I am wrong.) What I am confused is that in the Z+1 approximation, the input file specifies the number of electron to be 8*4=32. Since the special carbon atom has valence 5 (instead of 4), the system is charged. However, my understanding of x-ray absorption is that an incoming photon excites an electron from 1s state into the conduction band. If that is the case, then why not run a charge neutral calculation, i.e. nele = 7*4+5=33, and the resulting system is metallic?
>     Thank you.
> 
> Hanghui Chen
> Department of Physics 
> Yale University 
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Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

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