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

[Lorenzo Bastonero]

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.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
> <1698160383227blob.jpg>
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