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<div dir="ltr" data-setdir="false">Dear Nicola,</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">I read the paper, but I still don't understand about <span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">the difference between primitive-cell calculations and density-functional perturbation theory at any arbitrary q wavevector.</span></span></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><br></span></span></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">If I am only interesting in Gamma point (which q = 0, and other points in <span>Brillouin zone not interested.</span>), would it be ok to calculate Raman Spectra with only 1 unit cell, or do I need more unit cells to satisfy it?</span></span></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><br></span></span></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><span>Best Regards, </span></span></span></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><span><br></span></span></span></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><span>HY Lu</span><br></span></span></div><div dir="ltr" data-setdir="false"><span><div><blockquote type="cite" style="margin: 0px; padding: 8px; color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;"><div dir="ltr"><div id="ydp74a43d92yiv8406788056yqtfd79419" class="ydp74a43d92yiv8406788056yqt0953615191"><div class="ydp74a43d92yiv8406788056ydp8b3f45bbyahoo-style-wrap"><div dir="ltr"><div><br></div><div><br></div><div><br></div></div></div></div></div></blockquote></div></span></div><div><br></div>
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在 2023年11月13日 星期一 上午04:29:48 [GMT+8]， Nicola Marzari<nicola.marzari@epfl.ch> 寫道：
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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>http://theossrv1.epfl.ch/Main/Contact</div>
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<blockquote type="cite">On 12 Nov 2023, at 17:29, ludwigboltzmann.sc10@nycu.edu.tw wrote:<br clear="none">
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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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</span><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]， <mkondrin@hppi.troitsk.ru> 寫道： </div>
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</div><div dir="ltr"><div id="ydp74a43d92yiv8406788056yqtfd55252" class="ydp74a43d92yiv8406788056yqt0953615191">Hi, Ludwig<br clear="none">
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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">
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Sincerely yours,<br clear="none">
M.V.Kondrin<br clear="none">
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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">
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> 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:https://www.quantum-espresso.org/wp-content/uploads/2022/03/phonons_tutorial_shanghai1.pdf<br clear="none">
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> 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</div><br clear="none">
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