<html><head></head><body><div class="yahoo-style-wrap" style="font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:13px;"><div dir="ltr" data-setdir="false"><span>Dear users:</span></div><div dir="ltr" data-setdir="false"><span><br></span></div><div dir="ltr" data-setdir="false">I am going to calculate Raman spectra for organic perovskite. (only at Gamma point)</div><div dir="ltr" data-setdir="false">My flow chart is like this： </div><div dir="ltr" data-setdir="false">pw.x vc-relax(geometry optimization)→pw.x scf→ph.x→dynmat.x</div><div dir="ltr" data-setdir="false"><span>In Phonon Calculation for Raman Spectra , How to decide to use how many unit cells?</span><br></div><div dir="ltr" data-setdir="false"><span><br></span></div><div dir="ltr" data-setdir="false">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 Neue", 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" data-setdir="false">Ref:</div><div dir="ltr" data-setdir="false"><a 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></span></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">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 Neue", Helvetica, Arial, sans-serif;">Raman spectra with on one unit cell?</span></span></div><div dir="ltr" data-setdir="false">Difference I could imagine is that if I had 1unit cell with 24 atoms,</div><div dir="ltr" data-setdir="false">With 1 unit cell (<span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;">24 atoms)</span></span>, I can get 72(3*24) modes at Gamma point. </div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;">With 2 unit cell (48</span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;"> atoms)</span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;">, I can get 144(3*48) modes at Gamma point. </span></span><br></div><div dir="ltr" data-setdir="false"><span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;"><span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; outline: none !important;">With 4 unit cell (96</span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; outline: none !important;"> atoms)</span><span style="color: rgb(0, 0, 0); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; outline: none !important;">, I can get 288(3*96) modes at Gamma point. <br></span></span>As the unit cell increased, I would get more modes at Gamma point.</span></span></div><div dir="ltr" data-setdir="false">However, it takes more time to do the calculation.</div><div dir="ltr" data-setdir="false">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" data-setdir="false"><br></div><div dir="ltr" data-setdir="false">Best Regards,</div><div dir="ltr" data-setdir="false">HY Lu</div></div></body></html>