<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Dear Hy Lu,<div><br></div><div>Gamma phonons are obtained within the unit cell only, so no need of supercells (for the neighborhood you add the non-analytical correction, so to account for long range interactions; see eg Gonze 1997 PRB or Baroni 2001 Rev Mod Phys).</div><div><br></div><div>As it is a perovskite structure though, I guess you might have some tilting of octahedra, which is something not accounted for in the unit cell, thus I assume the need of supercell (although I admit I didn’t read the references you sent).</div><div><br></div><div>If the latter is the case, then it really depends a lot on the material. Moreover, if you are trying to match some experiments you would need to carefully understand the experimental conditions and model your structure accordingly.</div><div><br></div><div>I think you can start with the unit cell and work your understanding of the problem up. The use of DFPT (ph.x code) will allow you to compute the frequencies in a cheap and user friendly manner.</div><div><br></div><div>Note that no temperature effects are included in the harmonic approximation (“bare” phonons).</div><div><br></div><div>HTH,</div><div>Lorenzo Bastonero</div><div><br id="lineBreakAtBeginningOfSignature"><div dir="ltr">Inviato da iPhone</div><div dir="ltr"><br><blockquote type="cite">Il giorno 24 ott 2023, alle ore 17:31, ludwigboltzmann.sc10@nycu.edu.tw ha scritto:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><div class="ydp7c83d9aeyahoo-style-wrap" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;"><div></div>
<div dir="ltr" data-setdir="false"><div><div dir="ltr" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif;" data-setdir="false">"<span>We have 144 normal modes, corresponding to thenumber of degrees of freedom of 4 CH3NH3PbI3units in ourunit cell" </span></div><div dir="ltr" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><br></div><div dir="ltr" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><br></div><div dir="ltr" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">Ref:1. <a href="https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.5b07432" rel="nofollow" target="_blank">https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.5b07432</a></div><div dir="ltr" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">----------------------------------------------------</div><div dir="ltr" style="font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">2.<a href="https://scholar.google.com/citations?view_op=view_citation&hl=en&user=4Ncn-GUAAAAJ&sortby=pubdate&citation_for_view=4Ncn-GUAAAAJ:9yKSN-GCB0IC" style="color: rgb(25, 106, 212); text-decoration-line: underline;" rel="nofollow" target="_blank">https://scholar.google.com/citations?view_op=view_citation&hl=en&user=4Ncn-GUAAAAJ&sortby=pubdate&citation_for_view=4Ncn-GUAAAAJ:9yKSN-GCB0IC</a></div></div><br></div><div><br></div>
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在 2023年10月24日 星期二 下午11:25:48 [GMT+8]， ludwigboltzmann.sc10@nycu.edu.tw<ludwigboltzmann.sc10@nycu.edu.tw> 寫道：
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<div><div id="ydpddcdb1b2yiv1678091906"><div><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;" class="ydpddcdb1b2yiv1678091906yahoo-style-wrap"><div dir="ltr">Dear QE users:</div><div dir="ltr"><span>I have a naive question of how to determine the number of unit cell to I need to calculate vibrational frequencies.(at Gamma point)</span><br></div><div dir="ltr"><span>In the below 2 paper, I saw authors choosing 4 unit cells of perovskite (MAPbI3) for phonon calculation.</span></div><div dir="ltr"><span><br></span></div><div dir="ltr">However, I do not know what number of unit cell should I use? Is their any way to determine it ?</div><div dir="ltr"><br></div><div dir="ltr"><div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">Sincerely,</div><div style="font-family:Helvetica, Arial, sans-serif;"><br></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">National Yang Ming Chiao Tung University<br></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">HY Lu</div></div><br></div><div dir="ltr"><span><br></span></div><div dir="ltr"><span><br></span></div><div dir="ltr"><div><1698160383227blob.jpg></div><br>Ref:1. <a href="https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.5b07432" rel="nofollow" target="_blank">https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.5b07432</a></div><div dir="ltr">2.<a href="https://scholar.google.com/citations?view_op=view_citation&hl=en&user=4Ncn-GUAAAAJ&sortby=pubdate&citation_for_view=4Ncn-GUAAAAJ:9yKSN-GCB0IC" rel="nofollow" target="_blank">https://scholar.google.com/citations?view_op=view_citation&hl=en&user=4Ncn-GUAAAAJ&sortby=pubdate&citation_for_view=4Ncn-GUAAAAJ:9yKSN-GCB0IC</a></div></div></div></div></div>
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