[Pw_forum] negative values in XANES (jiayudai)
matteo calandra
matteo.calandra at impmc.jussieu.fr
Thu Sep 30 15:50:01 CEST 2010
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" <daijiayu at nudt.edu.cn>
> Subject: Re: [Pw_forum] negative values in XANES
> To: pw_forum at pwscf.org
> Message-ID: <BFLMVYOEFMYQVOJQQPWVTQFEHNFV.daijiayu at nudt.edu.cn>
> 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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> morale de la pens?e - Jean Piaget
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> ------------------------------
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> Message: 4
> Date: Thu, 30 Sep 2010 21:03:06 +0800
> From: "archygu" <archygu at gmail.com>
> Subject: [Pw_forum] the keyword in the pw.x
> To: pw_forum <pw_forum at pwscf.org>
> Message-ID: <201009302103032657359 at gmail.com>
> Content-Type: text/plain; charset="gb2312"
>
> Dear QE Users,
>
> As a beginner of QE, I get problem in understanding some keywords in
> PW.x, it will be very kind of you to give me a more
>
> detailed explanation:
>
> nbnd: acturally I do not know how to use this parameter , sometimes,
> a lot of levels appear in the valence bands, but only several in the
> conduction band.
> so I want to know how can I accurately set the parameter,
> then can obtain the energy levels properly.
>
> nosym: if it is true, that means the symmetry in the structure will
> be ignored?
>
>
>
> nraise: how can we define one "nraise" step, it is the same as one
> time step if we set nraise=1;
>
> many thanks for you help.
>
>
> 2010-09-30
>
>
>
> archygu,NTU
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