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<div dir="ltr"><div><blockquote type="cite"><div dir="ltr"><div id="ydpb00cf2c9yiv5602044312ydp2b93bebfyiv8570436949ydpddcdb1b2yahoo_quoted_8766153154" class="ydpb00cf2c9yiv5602044312ydp2b93bebfyiv8570436949ydpddcdb1b2yahoo_quoted"><div style="color:rgb(38, 40, 42);"><div id="ydpb00cf2c9yiv5602044312ydp2b93bebfyiv8570436949ydpddcdb1b2yiv1678091906"><div class="ydpb00cf2c9yiv5602044312ydp2b93bebfyiv8570436949ydpddcdb1b2yiv1678091906yahoo-style-wrap"><div dir="ltr"><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;"><span><span style="color:rgb(38, 40, 42);font-family:Helvetica Neue, Helvetica, Arial, sans-serif;">Dear Lorenzo Bastonero,</span></span><br clear="none"></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;"><span><span style="color:rgb(38, 40, 42);font-family:Helvetica Neue, Helvetica, Arial, sans-serif;"><br clear="none"></span></span></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;"><span><span style="color:rgb(38, 40, 42);font-family:Helvetica Neue, Helvetica, Arial, sans-serif;">Thanks for the informative paper, I'll read it.</span></span></div><div dir="ltr">I'm not sure I totally understand the answer, so do that mean I need to try from 1 →2→4 →8 unit cells?</div><div dir="ltr">Actually, I'm not sure what I would see the difference (I know the phonon mode is 3*N, and it would surely change) and when to stop, and use that number of cell (since more atoms costs more)</div><div dir="ltr">Sorry for the naive question.</div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;"><br clear="none"></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">Sincerely,</div><div style="font-family:Helvetica, Arial, sans-serif;"><br clear="none"></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">National Yang Ming Chiao Tung University<br clear="none"></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">HY Lu</div></div></div></div></div></div></div></blockquote></div><br clear="none"></div><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;"><br clear="none"></div>
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在 2023年10月25日 星期三 上午12:31:10 [GMT+8]， Lorenzo Bastonero<lbastone@uni-bremen.de> 寫道：
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<div id="ydpb00cf2c9yiv5602044312yqt90356" class="ydpb00cf2c9yiv5602044312yqt5376516465"><div><div id="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949"><div>Dear Hy Lu,<div><br clear="none"></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 clear="none"></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 clear="none"></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 clear="none"></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 clear="none"></div><div>Note that no temperature effects are included in the harmonic approximation (“bare” phonons).</div><div><br clear="none"></div><div>HTH,</div><div>Lorenzo Bastonero</div><div><br clear="none" id="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949lineBreakAtBeginningOfSignature"><div dir="ltr">Inviato da iPhone</div><div dir="ltr"><br clear="none"><blockquote type="cite">Il giorno 24 ott 2023, alle ore 17:31, ludwigboltzmann.sc10@nycu.edu.tw ha scritto:<br clear="none"><br clear="none"></blockquote></div><blockquote type="cite"><div dir="ltr"><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;" class="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949ydp7c83d9aeyahoo-style-wrap"><div></div>
<div dir="ltr"><div><div dir="ltr" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;">"<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 clear="none"></div><div dir="ltr" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;"><br clear="none"></div><div dir="ltr" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;">Ref:1. <a shape="rect" 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 shape="rect" 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 clear="none"></div><div><br clear="none"></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="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949ydpddcdb1b2yiv1678091906"><div><div style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;" class="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949ydpddcdb1b2yiv1678091906yahoo-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 clear="none"></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 clear="none"></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 clear="none"></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 clear="none"></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">National Yang Ming Chiao Tung University<br clear="none"></div><div dir="ltr" style="font-family:Helvetica, Arial, sans-serif;">HY Lu</div></div><br clear="none"></div><div dir="ltr"><span><br clear="none"></span></div><div dir="ltr"><span><br clear="none"></span></div><div dir="ltr"><div><1698160383227blob.jpg></div><div id="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949yqtfd98564" class="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949yqt0224649974"><br clear="none">Ref:1. <a shape="rect" 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><div id="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949yqtfd15293" class="ydpb00cf2c9yiv5602044312ydpcea8dd5fyiv8570436949yqt0224649974"><div dir="ltr">2.<a shape="rect" 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></div>
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