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

[mkondrin at hppi.troitsk.ru]

Hi, Ludwig

Raman modes should be calculate in primitive unit cell so you get the 
correct number of phonon modes. In silicon you get six modes -- triply 
degenerate Raman mode and three acoustic modes.

Sincerely yours,
M.V.Kondrin
 
On Nov 12 2023, ludwigboltzmann.sc10 at nycu.edu.tw wrote:

> 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