[Pw_forum] XSpectrum

[Hanghui Chen]

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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