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

[ludwigboltzmann.sc10 at nycu.edu.tw]

 Dear Ramesh,
Thank you for the paper, I read part of it.

Best Regards,
HY Lu


    在 2023年11月13日 星期一 下午03:20:31 [GMT+8]， Ramesh Kumar Kamadurai<rkamadur at gitam.edu> 寫道：  
 
 Dear Professor Nicola MarzariThank you for the reply. Yes I did not mention that the supercell calculations should be done using the Phonopy package. Still I am not very clear about the difference between these two methods (DFPT and Frozen Phonon) but I could complete 128 atoms supercell phonon dispersion with minimal computational resources using Phonopy. Dear HY LuIt is hard for me to tell why certain calculations use 2x2x1 unit cells instead of 1x1x1 unit cells. I will share an article which calculated the phonon dispersion of a 1x5x1 tetragonal cell instead of a 1x1x1 cubic unicell. The reason for doing such supercell calculation is explained here. Hope this will give you an idea why such symmetry reduced supercell calculation is being carried out. (Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys https://arxiv.org/abs/cond-mat/0506774)RegardsRamesh
On Mon, Nov 13, 2023 at 2:00 AM Nicola Marzari via users <users at lists.quantum-espresso.org> wrote:



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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