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<div class="">Dear QE friends and Paolo,</div>
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<div class="">I’m sorry to break into this mail thread, but could you elaborate a bit on “applying acoustic sum rule”.</div>
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<div class="">How is this done?</div>
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Met vriendelijke groeten,</div>
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Mit freundlichen Grüßen,</div>
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<blockquote type="cite" class="">
<div class="">On 14 Sep 2020, at 08:34, Paolo Giannozzi <<a href="mailto:p.giannozzi@gmail.com" class="">p.giannozzi@gmail.com</a>> wrote:</div>
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<div class="">Apparently the largest discrepancies come from low-energy modes, in particular zero-frequency acoustic modes (those with "negative" frequencies that become zero when acoustic sum rule is applied). Even minor changes in the way exchange-correlation
is computed may lead to surprisingly large but unphysical differences for those modes.<br class="">
</div>
<div class=""><br class="">
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<div class="">Paolo<br class="">
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<div dir="ltr" class="gmail_attr">On Sun, Sep 13, 2020 at 7:54 PM Unmesh Mondal <<a href="mailto:unmesh.mondal@students.iiserpune.ac.in" class="">unmesh.mondal@students.iiserpune.ac.in</a>> wrote:<br class="">
</div>
<blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;">
<div dir="ltr" class="">Dear all,
<div class="">
<div class="">I intend to calculate the vibrational modes of gas phase HCOOH (formic acid) molecule.</div>
<div class=""> <br class="">
</div>
<div class="">The "ph.x" vibrational frequencies (in cm-1) of the system for the two versions of quantum espresso (6.5 and 6.4 : with same compilers) are: </div>
<div class=""> (QE 6.5) (QE 6.4)</div>
<div class=""> 1 -207.654081 -207.686563 <br class="">
2 -63.775769 -61.840758 <br class="">
3 -55.090067 -57.158594 <br class="">
4 -15.183543 -15.903109 <br class="">
5 20.311666 21.386665 <br class="">
6 129.683418 129.214185 <br class="">
7 615.352300 615.209434 <br class="">
8 676.597853 676.547298 <br class="">
9 999.285772 999.331118 <br class="">
10 1071.840671 1071.779138 <br class="">
11 1253.496419 1253.514267 <br class="">
12 1350.618940 1350.925730 <br class="">
13 1735.698557 1735.582574 <br class="">
14 2965.285270 2965.885980 <br class="">
15 3665.961548 3665.948232 <br class="">
</div>
<div class=""><br class="">
</div>
<div class="">The maximum difference in any frequency for the two versions is about 2 cm-1. My question: </div>
<div class=""> Is the difference of 2 cm-1 within the error limit of ph.x calculations? What is the accuracy we expect in reporting the vibrational frequency (via ph.x)?</div>
<div class=""><br class="">
</div>
<div class="">Regards and Thanks</div>
<div class="">Unmesh Mondal</div>
<div class="">IISER Pune, India</div>
<div class=""><br class="">
</div>
</div>
</div>
_______________________________________________<br class="">
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<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">--<span class="Apple-converted-space"> </span></span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class="">
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<div class="">Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,<br class="">
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy<br class="">
Phone +39-0432-558216, fax +39-0432-558222<br class="">
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