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

[ludwigboltzmann.sc10 at nycu.edu.tw]

 Dear Mr. Kumar,
Thanks for your advice of using supercell calculation in doing phonon and Raman calculations.I checked that in the field I'm investigating in (perovskite), some literature are using 2x2x1(4 unit cells) for calculation, but some are using 1x1x1(1 unit cells)However, I didn't see why this number in the paper.Do you have any idea of choosing this size (4 or 1units) of cell? p.s. for cutoff energy, different cutoff energies may be apply to do geometry optimization and apply when the energy converges, but for unit cell size, I have no idea...
Best Regards,HY Lu


    在 2023年11月12日 星期日 下午01:12:32 [GMT+8]， Ramesh Kumar Kamadurai<rkamadur at gitam.edu> 寫道：  
 
 Dear HY Lu,

When deciding on the unit cell for phonon and Raman calculations, it's important to consider the material's properties rather than relying on general methods. Typically, if you're exploring interactions with meV excitations related to phonons (vibrational modes), using a supercell calculation is necessary for more accurate results. The same applies if you're investigating phenomena like CDW ordering or other ordering with commensurate/incommensurate lattice periodicity.

I recommend checking literature in your specific field of research for guidance, as examples from Quantum ESPRESSO or other material calculations may not directly apply. The choice of the unit cell tends to be material-specific and depends on the phenomena you're studying.

Best Regards,
Ramesh KumarAssistant ProfessorGITAM, Vizag, India
On Sun, Nov 12, 2023 at 9:10 AM ludwigboltzmann.sc10 at nycu.edu.tw <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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