[Pw_forum] negative values in XANES (jiayudai)

[Stefano Baroni]

Matteo, thank you for taking the time to participate in the discussion. In the future, please hit "reply" when answering a post, rather than starting a new thread. Have all a good day - Stefano B

On Sep 30, 2010, at 9:13 AM, matteo calandra wrote:

> 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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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
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