[Pw_forum] negative values in XANES (jiayu dai)
jiayudai
daijiayu at nudt.edu.cn
Sat Oct 2 10:18:17 CEST 2010
Dear matteo calandra,
Thanks for your instructions. I did the comparision as you said. The DOS and the xanes before occupied cutting can be comparable (I will send the files to you seperately). However, whatever i did, the xanes after occupied cutting was strange. I attached the files i used, maybe there was something error or lost information, but i never found them.
The strange thing was that even there are values larger than zero in the xanes before cutting, the xanes after cutting is most zero except in a narrow area in energy (x-aixs). I did not know where the values gone.
Thanks a lot，and best wishes.
Jiayu
Date: Thu, 30 Sep 2010 15:50:01 +0200
From: matteo calandra
Subject: Re: [Pw_forum] negative values in XANES (jiayudai)
To: pw_forum at pwscf.org
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Dear Jiayudai,
it is a bit difficult to understand what do you mean without having a
picture. Maybe you can try the following for the high pressure case.
You take a GIPAW pseudopotential compatible with XSPECTRA
without core hole and do again the calculation without cutting occupied
states but inserting your ef_r value. Then you compare your results with the
corresponding projected density of states calcolated with projwfc. For
corresponding I mean
corresponding to the choice of your polarization (and k-vector
if you are calculating the quadrupolar part).
They should be the same (or very similar as probably you don't have all
the same parameters in the two calculations, number of bands, broadening...)
except for a prefactor along the y axis.
So you can check if you are not making a mistake somewhere.
All the best, M.
>
> Message: 3
> Date: Thu, 30 Sep 2010 19:54:51 +0800
> From: "jiayudai"
> Subject: Re: [Pw_forum] negative values in XANES
> To: pw_forum at pwscf.org
> Message-ID:
> Content-Type: text/plain; charset="gb2312"
>
> 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
>
> Message-ID:
>
> Content-Type: text/plain; charset="iso-8859-1"
>
> 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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>>
>>
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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)
>
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> End of Pw_forum Digest, Vol 39, Issue 57
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> ----------------------------
----------------
-------------------------------------------
Jiayu Dai
Department of Physics
National University of Defense Technology,
Changsha, 410073, P R China
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