<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">Dear QE users:</div><div dir="ltr" data-setdir="false"><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" data-setdir="false"><span>In the below 2 paper, I saw authors choosing 4 unit cells of perovskite (MAPbI3) for phonon calculation.</span></div><div dir="ltr" data-setdir="false"><span><br></span></div><div dir="ltr" data-setdir="false">However, I do not know what number of unit cell should I use? Is their any way to determine it ?</div><div dir="ltr" data-setdir="false"><br></div><div dir="ltr" data-setdir="false"><div><div dir="ltr" style="font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;">Sincerely,</div><div style="font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;"><br></div><div dir="ltr" style="font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;">National Yang Ming Chiao Tung University<br></div><div dir="ltr" style="font-family: "Helvetica Neue", Helvetica, Arial, sans-serif;">HY Lu</div></div><br></div><div dir="ltr" data-setdir="false"><span><br></span></div><div dir="ltr" data-setdir="false"><span><br></span></div><div dir="ltr" data-setdir="false"><img title="內嵌影像" alt="內嵌影像" src="cid:6775c608-cf30-2b65-e16f-f2ac7ce87d76@yahoo.com" class="yahoo-inline-image" draggable="false" style="max-width: 608px; width: 100%;" data-id="<6775c608-cf30-2b65-e16f-f2ac7ce87d76@yahoo.com>"><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" data-setdir="false">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></body></html>