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

[ludwigboltzmann.sc10 at nycu.edu.tw]

Dear Lorenzo Bastonero,

Thanks for the informative paper, I'll read it.I'm not sure I totally understand the answer, so do that mean I need to  try from 1 →2→4 →8 unit cells?Actually, I'm not sure what I would see the difference (I know the phonon mode is 3*N, and it would surely change) and when to stop, and use that number of cell (since more atoms costs more)Sorry for the naive question.
Sincerely,
National Yang Ming Chiao Tung University
HY Lu


    在 2023年10月25日 星期三 上午12:31:10 [GMT+8]， Lorenzo Bastonero<lbastone at uni-bremen.de> 寫道：  
 
 Dear Hy Lu,
Gamma phonons are obtained within the unit cell only, so no need of supercells (for the neighborhood you add the non-analytical correction, so to account for long range interactions; see eg Gonze 1997 PRB or Baroni 2001 Rev Mod Phys).
As it is a perovskite structure though, I guess you might have some tilting of octahedra, which is something not accounted for in the unit cell, thus I assume the need of supercell (although I admit I didn’t read the references you sent).
If the latter is the case, then it really depends a lot on the material. Moreover, if you are trying to match some experiments you would need to carefully understand the experimental conditions and model your structure accordingly.
I think you can start with the unit cell and work your understanding of the problem up. The use of DFPT (ph.x code) will allow you to compute the frequencies in a cheap and user friendly manner.
Note that no temperature effects are included in the harmonic approximation (“bare” phonons).
HTH,Lorenzo Bastonero
Inviato da iPhone

Il giorno 24 ott 2023, alle ore 17:31, ludwigboltzmann.sc10 at nycu.edu.tw ha scritto:



 "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


<1698160383227blob.jpg>
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  <1698160383227blob.jpg>_______________________________________________
The Quantum ESPRESSO community stands by the Ukrainian
people and expresses its concerns about the devastating
effects that the Russian military offensive has on their
country and on the free and peaceful scientific, cultural,
and economic cooperation amongst peoples
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list users at lists.quantum-espresso.org
https://lists.quantum-espresso.org/mailman/listinfo/users
    
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20231024/4a870625/attachment.html>