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

[Ramesh Kumar Kamadurai]

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 Kumar
Assistant Professor
GITAM, 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.x
> In 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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