[Pw_forum] negative values in XANES

[jiayudai]

Dear matteo,

Thanks for your reply to me. The "negative" means the negative position in x-axis. And i added the "ef_r" in the input file, and the position of the absorption is reasonable now. However, when the pressure is high (high density), the fermi energy should be also very high. Even i added the "ef_r", the absorption after cutting the contribution of occupied states is only a isolated peak. In the same case but with lower density, there is not this problem. And the analysis of density of states for the system tells me that there should be absorption at the position of high energy. In this case, the absorption before occ_cut and after occ_cut is very different (the intensity is very weak, about 10-8). So, i can not find physcal reason to explain it. Is there also some problems in my calculation?

By the way, i changed some parameters of &occ_cut& part, but the results were the same. 

Thanks again, and best wishes.

Jiayu 


Subject: Re: [Pw_forum] negative values in XANES (jiayudai)
To: PWSCF Forum

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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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