<div dir="ltr">Dear Professor Nicola Marzari<div>Thank you for the reply. Yes I did not mention that the supercell calculations should be done using the Phonopy package. Still I am not very clear about the difference between these two methods (DFPT and Frozen Phonon) but I could complete 128 atoms supercell phonon dispersion with minimal computational resources using Phonopy. </div><div>Dear HY Lu</div><div>It is hard for me to tell why certain calculations use 2x2x1 unit cells instead of 1x1x1 unit cells. I will share an article which calculated the phonon dispersion of a 1x5x1 tetragonal cell instead of a 1x1x1 cubic unicell. The reason for doing such supercell calculation is explained here. Hope this will give you an idea why such symmetry reduced supercell calculation is being carried out. (Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys <a href="https://arxiv.org/abs/cond-mat/0506774">https://arxiv.org/abs/cond-mat/0506774</a>)</div><div>Regards</div><div>Ramesh</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Nov 13, 2023 at 2:00 AM Nicola Marzari via users <<a href="mailto:users@lists.quantum-espresso.org">users@lists.quantum-espresso.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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Dear all,
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<div>I think there is a profound misunderstanding here - make sure you understand the difference between primitive-cell calculations and density-functional perturbation theory at any arbitrary q wavevector (read 2001 RMP by Baroni et al) and finite-difference
calculations in a supercell, a-la phonopy.</div>
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<div>nicola</div>
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<div dir="ltr">Sent from a tiny keyboard... Contact info:
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<div><a href="http://theossrv1.epfl.ch/Main/Contact" target="_blank">http://theossrv1.epfl.ch/Main/Contact</a></div>
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<blockquote type="cite">On 12 Nov 2023, at 17:29, <a href="mailto:ludwigboltzmann.sc10@nycu.edu.tw" target="_blank">ludwigboltzmann.sc10@nycu.edu.tw</a> wrote:<br>
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<div dir="ltr">Dear Mr. <span><span style="color:rgb(38,40,42);font-family:"Helvetica Neue",Helvetica,Arial,sans-serif">Kondrin,</span></span></div>
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<div dir="ltr"><span><span style="color:rgb(38,40,42);font-family:"Helvetica Neue",Helvetica,Arial,sans-serif">Thanks, this is exactly for silicon. </span></span></div>
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<div><span style="color:rgb(38,40,42);font-family:"Helvetica Neue",Helvetica,Arial,sans-serif">Best</span><span style="color:rgb(38,40,42);font-family:"Helvetica Neue",Helvetica,Arial,sans-serif"> Regards,</span></div>
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<div><span style="color:rgb(38,40,42);font-family:"Helvetica Neue",Helvetica,Arial,sans-serif">HY Lu</span></div>
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<div>在 2023年11月12日 星期日 下午02:52:03 [GMT+8]， <<a href="mailto:mkondrin@hppi.troitsk.ru" target="_blank">mkondrin@hppi.troitsk.ru</a>> 寫道： </div>
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<div dir="ltr">Hi, Ludwig<br clear="none">
<br clear="none">
Raman modes should be calculate in primitive unit cell so you get the <br clear="none">
correct number of phonon modes. In silicon you get six modes -- triply <br clear="none">
degenerate Raman mode and three acoustic modes.<br clear="none">
<br clear="none">
Sincerely yours,<br clear="none">
M.V.Kondrin<br clear="none">
<div id="m_716793580519571501ydpbcaff425yqtfd52572"><br clear="none">
On Nov 12 2023, <a shape="rect" href="mailto:ludwigboltzmann.sc10@nycu.edu.tw" rel="nofollow" target="_blank">
ludwigboltzmann.sc10@nycu.edu.tw</a> wrote:<br clear="none">
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> Dear users: I am going to calculate Raman spectra for organic perovskite. <br clear="none">
> (only at Gamma point)My flow chart is like this： pw.x vc-relax(geometry <br clear="none">
> optimization)→pw.x scf→ph.x→dynmat.xIn Phonon Calculation for Raman <br clear="none">
> Spectra , How to decide to use how many unit cells?<br clear="none">
><br clear="none">
> I saw what in the tutorial, for silicon in primitive cell, Si only has 2 <br clear="none">
> atoms in a primitive cell, and the tutorial used 2 atoms (1 unit cell to <br clear="none">
> calculate)Ref:<a href="https://www.quantum-espresso.org/wp-content/uploads/2022/03/phonons_tutorial_shanghai1.pdf" target="_blank">https://www.quantum-espresso.org/wp-content/uploads/2022/03/phonons_tutorial_shanghai1.pdf</a><br clear="none">
><br clear="none">
><br clear="none">
> However, according to the paper I read, the author used 4 unit cells to <br clear="none">
> calculate, is this going to make some difference? Is it ok to <br clear="none">
> calculate Raman spectra with on one unit cell?Difference I could imagine <br clear="none">
> is that if I had 1unit cell with 24 atoms,With 1 unit cell (24 atoms), I <br clear="none">
> can get 72(3*24) modes at Gamma point. With 2 unit cell (48 atoms), I can <br clear="none">
> get 144(3*48) modes at Gamma point. With 4 unit cell (96 atoms), I can <br clear="none">
> get 288(3*96) modes at Gamma point. As the unit cell increased, I would <br clear="none">
> get more modes at Gamma point.However, it takes more time to do the <br clear="none">
> calculation.I'm wondering what's the difference to do phonon calculation <br clear="none">
> to use different number of unit cells? And how to decide it? Best <br clear="none">
> Regards,HY Lu<br clear="none">
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