[Pw_forum] negative values in XANES (jiayudai)

[matteo calandra]

Dear Jiayu,

   I assume you're using XSPECTRA to calculate Xanes K-edge.
Maybe you can better specify what does it means "negative
absorption". If this means that the value on the XANES intensity is negative
then you must have some problem somewhere as the cross section
implemented in XSPECTRA is positive definite.

However, from your e-mail it is not very clear if the
absorption is positive definite (as you say) or if the energy scale
(the x axis in the plot)
is negative but not the X-ray absorption intensity (y-axis in the plot).
If y is positive and x is negative, then you simply forgot
to specify the variable
ef_r in the XSPECTRA input file. This variable tells you the zero
energy of the scale. From the XSPECTRA input file:

ef_r              real(DP)                                        DEFAULT=0.0
                   Fermi energy in Rydberg. This value combined
                   with the option cut_occupied_states can be
                   used to exclude the occupied states in a
                   smooth way from the final plot.

I hope it helps,

  M.


> Dear developers and users,
>
> I am doing the calculation for the XANES of CHx system. There is a  
> problem that the absorption is always negative, locating at about  
> -100 eV. According to my >understanding, the absorption should be  
> positive. Besides, the fermi energy in >my systems is very high,  
> about tens of eV (high pressure). Therefore, if i used the cutoff of  
> fermi energy, the absorption should be very small. Is it  
> >reasonable? Or there is some problems in the calculations?
>
> Thanks in advance.
>
> Jiayu

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