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

[ludwigboltzmann.sc10 at nycu.edu.tw]

 Dear Nicola,
I read the paper, but I still don't understand about the difference between primitive-cell calculations and density-functional perturbation theory at any arbitrary q wavevector.
If I am only interesting in Gamma point (which q = 0, and other points in Brillouin zone not interested.), would it be ok to calculate Raman Spectra with only 1 unit cell, or do I need more unit cells to satisfy it?
Best Regards, 
HY Lu






    在 2023年11月13日 星期一 上午04:29:48 [GMT+8]， Nicola Marzari<nicola.marzari at epfl.ch> 寫道：  
 
 

Dear all,
I think there is a profound misunderstanding here - make sure you understand the difference between primitive-cell calculations and density-functional perturbation theory at any arbitrary q wavevector (read 2001 RMP by Baroni et al) and finite-difference calculations in a supercell, a-la phonopy.
nicola

Sent from a tiny keyboard... Contact info:http://theossrv1.epfl.ch/Main/Contact

On 12 Nov 2023, at 17:29, ludwigboltzmann.sc10 at nycu.edu.tw wrote:



Dear Mr. Kondrin,
Thanks, this is exactly for silicon. 
Best Regards,
HY Lu
在 2023年11月12日 星期日 下午02:52:03 [GMT+8]， <mkondrin at hppi.troitsk.ru> 寫道： 

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

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