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, [email protected] 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]， > [email protected]<[email protected]> 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> > _______________________________________________ > 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 [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users
_______________________________________________ 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 [email protected] https://lists.quantum-espresso.org/mailman/listinfo/users
