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

[Ramesh Kumar Kamadurai]

Dear Professor Nicola Marzari
Thank 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 Lu
It 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)
Regards
Ramesh

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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