[Pw_forum] different computing cores lead to different results when calculating phonon spectrum by ph.x
Paolo Giannozzi
p.giannozzi at gmail.com
Mon Sep 12 22:17:50 CEST 2016
On Mon, Sep 12, 2016 at 7:43 PM, XIAOMING ZHANG <xiaom.zhang at utah.edu>
wrote:
do you know the reason why different computing cores lead to different
> results
At q=0, you should get three zero frequencies. You don't, because the
calculation is approximate and violates the so-called Acoustic Sum Rule,
that is, translational invariance. The main reason for such violation is
the usage of a real-space grid in the calculation of the
exchange-correlation potential, causing an aliasing error. The nonzero
value of the "zero" frequency is unpredictable: all you know is that it is
small (smaller for LDA than for GGA, almost zero if the XC potential is
zero, but this is not very useful anyway). It depends upon the number of
computing cores, upon the kind of parallelization, upon the compiler, the
libraries, the phase of the moon, ...
And How to fix it? I really confusing about this.
>
if you find how to fix it, you can write a nice paper.
Paolo
> Thanks for your time,
> Xiaoming
> Department of Materials Science and Engineering,
> University of Utah
>
>
> ------------------------------
> *From:* pw_forum-bounces at pwscf.org [pw_forum-bounces at pwscf.org] on behalf
> of Paolo Giannozzi [p.giannozzi at gmail.com]
> *Sent:* Sunday, September 11, 2016 12:51 PM
> *To:* PWSCF Forum
> *Subject:* Re: [Pw_forum] different computing cores lead to different
> results when calculating phonon spectrum by ph.x
>
> The frequencies you report are 0 by definition, almost 0 when computed:
> http://www.quantum-espresso.org/faq/phonons/#7.2
> <http://redir.aspx?REF=4sg-3oovjh9dbihtpRKtW8iIgpzbkIFFdTzPFukAKJtulrBgM9vTCAFodHRwOi8vd3d3LnF1YW50dW0tZXNwcmVzc28ub3JnL2ZhcS9waG9ub25zLyM3LjI.>
> . No available solution other than imposing the Acoustic Sum Rule (ASR)
> afterwards.
> Basically, the ASR violation is numerical noise, and as such, rather
> unpredictable.
>
> Paolo
>
> On Sun, Sep 11, 2016 at 8:10 PM, XIAOMING ZHANG <xiaom.zhang at utah.edu
> <http://redir.aspx?REF=rcdX65FVruQA1h4tbMGDKMD1AbCtgbuH3RWw8G8Mz0pulrBgM9vTCAFtYWlsdG86eGlhb20uemhhbmdAdXRhaC5lZHU.>
> > wrote:
>
>> Dear all,
>>
>> I have several questions when calculating phonons by ph.x:
>>
>> (1) Using the same input files, I got different output files of dynamical matrices when using different computing cores;
>>
>> take q = ( 0.000000000 0.000000000 0.000000000 ) as an example:
>>
>> Using 96 computing cores, the results after the line "Dynamical Matrix in cartesian axes"
>>
>> and the q-value are:
>>
>> **************************************************************************
>> freq ( 1) = -1.940214 [THz] = -64.718584 [cm-1]
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.237593 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.237593 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.237593 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.237593 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.237593 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.237593 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.209687 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.209687 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.209687 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.209687 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.209687 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.209687 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.363998 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.363998 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.363998 0.000000 )
>> freq ( 2) = 0.946381 [THz] = 31.567870 [cm-1]
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.101981 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.101981 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.101979 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.101979 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.000001 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.000001 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.384498 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 -0.384498 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.384493 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.384493 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.000005 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.000005 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 0.428402 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 -0.428397 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.000006 0.000000 )
>> freq ( 3) = 0.946381 [THz] = 31.567870 [cm-1]
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.058877 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.058877 0.000000 )
>> ( 0.000000 0.000000 -0.000000 0.000000 -0.058879 0.000000 )
>>
>> However, when I using 128 computing cores, the corresponding results turn to be:
>>
>> **************************************************************************
>> freq ( 1) = -1.941478 [THz] = -64.760743 [cm-1]
>> ( -0.000000 0.000000 -0.000000 0.000000 0.237504 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.237504 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.237504 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.237504 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.237504 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.237504 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.209612 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.209612 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.209612 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.209612 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.209612 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.209612 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.364201 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.364201 0.000000 )
>> ( -0.000000 0.000000 -0.000000 0.000000 0.364201 0.000000 )
>> freq ( 2) = -0.797967 [THz] = -26.617308 [cm-1]
>> ( -0.217955 0.000000 -0.132999 0.000000 -0.000000 0.000000 )
>> ( -0.217955 0.000000 -0.132999 0.000000 -0.000000 0.000000 )
>> ( -0.212134 0.000000 -0.133209 0.000000 0.000000 0.000000 )
>> ( -0.212134 0.000000 -0.133209 0.000000 0.000000 0.000000 )
>> ( -0.215227 0.000000 -0.138145 0.000000 0.000000 0.000000 )
>> ( -0.215227 0.000000 -0.138145 0.000000 0.000000 0.000000 )
>> ( -0.230258 0.000000 -0.129133 0.000000 -0.000000 0.000000 )
>> ( -0.230258 0.000000 -0.129133 0.000000 -0.000000 0.000000 )
>> ( -0.205571 0.000000 -0.130026 0.000000 0.000000 0.000000 )
>> ( -0.205571 0.000000 -0.130026 0.000000 0.000000 0.000000 )
>> ( -0.218687 0.000000 -0.150959 0.000000 0.000000 0.000000 )
>> ( -0.218687 0.000000 -0.150959 0.000000 0.000000 0.000000 )
>> ( -0.228822 0.000000 -0.140474 0.000000 0.000000 0.000000 )
>> ( -0.224086 0.000000 -0.140645 0.000000 -0.000000 0.000000 )
>> ( -0.226602 0.000000 -0.144661 0.000000 -0.000000 0.000000 )
>> freq ( 3) = -0.797967 [THz] = -26.617308 [cm-1]
>> ( -0.136570 0.000000 0.212256 0.000000 -0.000000 0.000000 )
>> ( -0.136570 0.000000 0.212256 0.000000 -0.000000 0.000000 )
>> ( -0.136360 0.000000 0.218077 0.000000 0.000000 0.000000 )
>>
>> So, why do different computing cores lead to different results? Does someone encounter the same problems?
>>
>> By the way, is it reasonable for the negative frequency (such as freq ( 1) = -1.940214 [THz] = -64.718584 [cm-1]) ?
>>
>>
>> (2) With different computing cores, I always get the phonon spectrum with small imaginary frequency no matter how I adjust the parameters suggested by other guys.
>>
>> So can someone give me some useful suggestions to eliminate the imaginary frequency?
>>
>>
>> Thanks for your time,
>>
>> Xiaoming
>>
>> Department of Materials Science and Engineering, University of Utah
>>
>>
>>
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>>
>
>
>
> --
> Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
> _______________________________________________
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>
--
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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