[QE-users] Naive question of choosing the number of unit cell to calculate vibrational frequencies

[ludwigboltzmann.sc10 at nycu.edu.tw]

 "We have 144 normal modes, corresponding to thenumber of degrees of freedom of 4 CH3NH3PbI3units in ourunit cell" 

Ref:1. https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.5b07432----------------------------------------------------2.https://scholar.google.com/citations?view_op=view_citation&hl=en&user=4Ncn-GUAAAAJ&sortby=pubdate&citation_for_view=4Ncn-GUAAAAJ:9yKSN-GCB0IC

    在 2023年10月24日 星期二 下午11:25:48 [GMT+8]， ludwigboltzmann.sc10 at nycu.edu.tw<ludwigboltzmann.sc10 at nycu.edu.tw> 寫道：  
 
 Dear QE users:I have a naive question of how to determine the number of unit cell to I need to calculate vibrational frequencies.(at Gamma point)
In the below 2 paper, I saw authors choosing 4 unit cells of perovskite (MAPbI3) for phonon calculation.
However, I do not know what number of unit cell should I use? Is their any way to determine it ?
Sincerely,
National Yang Ming Chiao Tung University
HY Lu



Ref:1. https://pubs.acs.org/doi/epdf/10.1021/acs.jpcc.5b074322.https://scholar.google.com/citations?view_op=view_citation&hl=en&user=4Ncn-GUAAAAJ&sortby=pubdate&citation_for_view=4Ncn-GUAAAAJ:9yKSN-GCB0IC  
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