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<div dir="ltr" data-setdir="false">Dear Mr. <span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">Kumar,</span></span></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">Thanks for your advice of using <span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">supercell calculation in doing <span><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">phonon and Raman calculations.</span></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><span style="color: rgb(38, 40, 42); font-family: Helvetica Neue, Helvetica, Arial, sans-serif;">I checked that in the field I'm investigating in (perovskite), some literature are using 2x2x1(4 unit cells) for calculation, but some are using 1x1x1(1 unit cells)</span></span></span></span></div><div dir="ltr" data-setdir="false"><font color="#26282a">However, I didn't see why this number in the paper.</font></div><div dir="ltr" data-setdir="false"><font color="#26282a">Do you have any idea of choosing this size (4 or 1units) of cell? </font></div><div dir="ltr" data-setdir="false"><font color="#26282a">p.s. for cutoff energy, different cutoff energies may be apply to do geometry optimization and apply when the energy converges, but for unit cell size, I have no idea...</font></div><div dir="ltr" data-setdir="false"><font color="#26282a"><br></font></div><div dir="ltr" data-setdir="false"><font color="#26282a"><div><div dir="ltr" style="color: rgb(38, 40, 42); font-family: Helvetica, Arial, sans-serif;">Best Regards,</div><div dir="ltr" style="color: rgb(38, 40, 42); font-family: Helvetica, Arial, sans-serif;">HY Lu</div></div><br></font></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><br></div>
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在 2023年11月12日 星期日 下午01:12:32 [GMT+8]， Ramesh Kumar Kamadurai<rkamadur@gitam.edu> 寫道：
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<div><div id="ydpb4d2a8e6yiv5344848445"><div><div dir="ltr">Dear HY Lu,<br clear="none"><br clear="none">When deciding on the unit cell for phonon and Raman calculations, it's important to consider the material's properties rather than relying on general methods. Typically, if you're exploring interactions with meV excitations related to phonons (vibrational modes), using a supercell calculation is necessary for more accurate results. The same applies if you're investigating phenomena like CDW ordering or other ordering with commensurate/incommensurate lattice periodicity.<br clear="none"><br clear="none">I recommend checking literature in your specific field of research for guidance, as examples from Quantum ESPRESSO or other material calculations may not directly apply. The choice of the unit cell tends to be material-specific and depends on the phenomena you're studying.<br clear="none"><br clear="none">Best Regards,<br clear="none">Ramesh Kumar<div>Assistant Professor</div><div>GITAM, Vizag, India</div></div><br clear="none"><div class="ydpb4d2a8e6yiv5344848445gmail_quote"><div id="ydpb4d2a8e6yiv5344848445yqt16714" class="ydpb4d2a8e6yiv5344848445yqt2827961182"><div dir="ltr" class="ydpb4d2a8e6yiv5344848445gmail_attr">On Sun, Nov 12, 2023 at 9:10 AM <a shape="rect" href="mailto:ludwigboltzmann.sc10@nycu.edu.tw" rel="nofollow" target="_blank">ludwigboltzmann.sc10@nycu.edu.tw</a> <<a shape="rect" href="mailto:ludwigboltzmann.sc10@nycu.edu.tw" rel="nofollow" target="_blank">ludwigboltzmann.sc10@nycu.edu.tw</a>> wrote:<br clear="none"></div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex;" class="ydpb4d2a8e6yiv5344848445gmail_quote"><div><div style="font-family:Helvetica, Arial, sans-serif;font-size:13px;"><div dir="ltr"><span>Dear users:</span></div><div dir="ltr"><span><br clear="none"></span></div><div dir="ltr">I am going to calculate Raman spectra for organic perovskite. (only at Gamma point)</div><div dir="ltr">My flow chart is like this： </div><div dir="ltr">pw.x vc-relax(geometry optimization)→pw.x scf→ph.x→dynmat.x</div><div dir="ltr"><span>In Phonon Calculation for Raman Spectra , How to decide to use how many unit cells?</span><br clear="none"></div><div dir="ltr"><span><br clear="none"></span></div><div dir="ltr">I saw what in the tutorial, for silicon in primitive cell, Si only has 2 atoms <span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;">in a primitive cell</span></span>, and the tutorial used 2 atoms (1 unit cell to calculate)</div><div dir="ltr">Ref:</div><div dir="ltr"><a shape="rect" href="https://www.quantum-espresso.org/wp-content/uploads/2022/03/phonons_tutorial_shanghai1.pdf" rel="nofollow" target="_blank">https://www.quantum-espresso.org/wp-content/uploads/2022/03/phonons_tutorial_shanghai1.pdf</a><span><br clear="none"></span></div><div dir="ltr"><br clear="none"></div><div dir="ltr"><br clear="none"></div><div dir="ltr">However, according to the paper I read, the author used 4 unit cells to calculate, is this going to make some difference? Is it ok to calculate <span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;">Raman spectra with on one unit cell?</span></span></div><div dir="ltr">Difference I could imagine is that if I had 1unit cell with 24 atoms,</div><div dir="ltr">With 1 unit cell (<span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;">24 atoms)</span></span>, I can get 72(3*24) modes at Gamma point. </div><div dir="ltr"><span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;">With 2 unit cell (48</span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;"> atoms)</span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;">, I can get 144(3*48) modes at Gamma point. </span></span><br clear="none"></div><div dir="ltr"><span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;"><span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;outline:none;">With 4 unit cell (96</span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;outline:none;"> atoms)</span><span style="color:rgb(0,0,0);font-family:Helvetica, Arial, sans-serif;outline:none;">, I can get 288(3*96) modes at Gamma point. <br clear="none"></span></span>As the unit cell increased, I would get more modes at Gamma point.</span></span></div><div dir="ltr">However, it takes more time to do the calculation.</div><div dir="ltr">I'm wondering what's the difference to do phonon calculation to use different number of unit cells? And how to decide it?</div><div dir="ltr"><br clear="none"></div><div dir="ltr">Best Regards,</div><div dir="ltr">HY Lu</div></div></div>_______________________________________________<br clear="none">
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