[QE-users] In Phonon Calculation for Raman Spectra, How to decide to use how many unit cells?

[ludwigboltzmann.sc10 at nycu.edu.tw]

Dear users:
I am going to calculate Raman spectra for organic perovskite. (only at Gamma point)My flow chart is like this： pw.x vc-relax(geometry optimization)→pw.x scf→ph.x→dynmat.xIn Phonon Calculation for Raman Spectra , How to decide to use how many unit cells?

I saw what in the tutorial, for silicon in primitive cell, Si only has 2 atoms in a primitive cell, and the tutorial used 2 atoms (1 unit cell to calculate)Ref:https://www.quantum-espresso.org/wp-content/uploads/2022/03/phonons_tutorial_shanghai1.pdf


However, according to the paper I read, the author used 4 unit cells to calculate, is this going to make some difference? Is it ok to calculate Raman spectra with on one unit cell?Difference I could imagine is that if I had 1unit cell with 24 atoms,With 1 unit cell (24 atoms), I can get 72(3*24) modes at Gamma point. With 2 unit cell (48 atoms), I can get 144(3*48) modes at Gamma point. 
With 4 unit cell (96 atoms), I can get 288(3*96) modes at Gamma point. 
As the unit cell increased, I would get more modes at Gamma point.However, it takes more time to do the calculation.I'm wondering what's the difference to do phonon calculation to use different number of unit cells? And how to decide it?
Best Regards,HY Lu
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