<div dir="ltr">On Mon, Sep 12, 2016 at 7:43 PM, XIAOMING ZHANG <span dir="ltr"><<a target="_blank" href="mailto:xiaom.zhang@utah.edu">xiaom.zhang@utah.edu</a>></span> wrote:<br><br><div class="gmail_extra"><div class="gmail_quote"><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex" class="gmail_quote"><font face="monospace" size="3">do you know the reason why different computing cores lead to different results </font></blockquote><div> <br>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, ...<br><br></div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex" class="gmail_quote"><div><div><div style="direction:ltr;font-family:tahoma;color:rgb(0,0,0);font-size:10pt"> <font face="monospace" size="3">And How to fix it? I really confusing about this.<br></font></div></div></div></blockquote><div> <br>if you find how to fix it, you can write a nice paper.<br><br>Paolo <br><br></div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex" class="gmail_quote"><div><div style="direction:ltr;font-family:tahoma;color:rgb(0,0,0);font-size:10pt"><font face="monospace" size="3">
<br>
Thanks for your time,<br>
Xiaoming<br>
Department of Materials Science and Engineering, </font>
<div><font face="monospace" size="3">University of Utah</font>
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<div style="direction:ltr"><font face="Tahoma" color="#000000" size="2"><b>From:</b> <a target="_blank" href="mailto:pw_forum-bounces@pwscf.org">pw_forum-bounces@pwscf.org</a> [<a target="_blank" href="mailto:pw_forum-bounces@pwscf.org">pw_forum-bounces@pwscf.org</a>] on behalf of Paolo Giannozzi [<a target="_blank" href="mailto:p.giannozzi@gmail.com">p.giannozzi@gmail.com</a>]<br>
<b>Sent:</b> Sunday, September 11, 2016 12:51 PM<br>
<b>To:</b> PWSCF Forum<br>
<b>Subject:</b> Re: [Pw_forum] different computing cores lead to different results when calculating phonon spectrum by ph.x<br>
</font><br>
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<div>The frequencies you report are 0 by definition, almost 0 when computed: <a target="_blank" href="http://redir.aspx?REF=4sg-3oovjh9dbihtpRKtW8iIgpzbkIFFdTzPFukAKJtulrBgM9vTCAFodHRwOi8vd3d3LnF1YW50dW0tZXNwcmVzc28ub3JnL2ZhcS9waG9ub25zLyM3LjI.">
http://www.quantum-espresso.<wbr>org/faq/phonons/#7.2</a> . No available solution other than imposing the Acoustic Sum Rule (ASR) afterwards.<br>
Basically, the ASR violation is numerical noise, and as such, rather unpredictable.<br>
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<div>Paolo<br>
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<div class="gmail_extra"><br>
<div class="gmail_quote">On Sun, Sep 11, 2016 at 8:10 PM, XIAOMING ZHANG <span dir="ltr">
<<a target="_blank" href="http://redir.aspx?REF=rcdX65FVruQA1h4tbMGDKMD1AbCtgbuH3RWw8G8Mz0pulrBgM9vTCAFtYWlsdG86eGlhb20uemhhbmdAdXRhaC5lZHU.">xiaom.zhang@utah.edu</a>></span> wrote:<br>
<blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex" class="gmail_quote">
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<pre>Dear all,</pre>
<pre>I have several questions when calculating phonons by ph.x:</pre>
<pre>(1) Using the same input files, I got different output files of dynamical matrices when using different computing cores; </pre>
<pre>take q = ( 0.000000000 0.000000000 0.000000000 ) as an example:</pre>
<pre>Using 96 computing cores, the results a<span style="font-size:10pt">fter the line "Dynamical Matrix in cartesian axes"</span></pre>
<pre>and the q-value are:</pre>
<pre> ******************************<wbr>******************************<wbr>**************
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 )</pre>
<pre>However, when I using 128 computing cores, the corresponding results turn to be:</pre>
<pre>******************************<wbr>******************************<wbr>**************
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 )</pre>
<pre>So, why do different computing cores lead to different results? Does someone encounter the same problems?</pre>
<pre>By the way, is it reasonable for the negative frequency (such as <span style="font-size:13.3333px">freq ( 1) = -1.940214 [THz] = -64.718584 [cm-1]</span><span style="font-size:10pt">) ?</span></pre>
<pre><span style="font-size:10pt"><br></span></pre>
<pre><span style="font-size:10pt">(2) With different computing cores, I always get the p</span>honon spectrum with small imaginary frequency no matter how I <span style="font-size:10pt">adjust the parameters suggested by other guys</span><span style="font-size:10pt">. </span></pre>
<pre><span style="font-size:10pt">S</span><span style="font-size:10pt">o can someone give me some useful suggestions to </span>eliminate<span style="font-size:10pt"> the </span><span style="font-size:13.3333px">imaginary frequency</span><span style="font-size:10pt">?</span></pre>
<pre><br></pre>
<pre>Thanks for <font size="3">your time,</font></pre>
<pre><font size="3">Xiaoming</font></pre>
<pre><span style="white-space:normal"><font size="3">Department of Materials Science and Engineering, University of Utah</font></span></pre>
<pre><span style="font-size:10pt"><br></span></pre>
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<div>Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,<br>
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy<br>
Phone <a target="_blank" value="+390432558216" href="tel:%2B39-0432-558216">+39-0432-558216</a>, fax <a target="_blank" value="+390432558222" href="tel:%2B39-0432-558222">+39-0432-558222</a><br>
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