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

[Lorenzo Paulatto]

> Sorry but for 1 more naive question,  if I use 2 unit cell instead of 
> 1 in DFPT, would it be expected to have difference between the two 
> cases ( only at Gamma point) ?

yes, it is different. Because you would get twice as many phonons, but 
half of them would be periodic inside the supercell (and correspond to 
Gamma) the other would not be (and correspond to a zone-border q-point).

hth


>
>
> Best Regards,
>
> HY Lu
>
>
>
>
>
> 在 2023年11月15日 星期三 下午03:56:04 [GMT+8]， Lorenzo 
> Paulatto<lorenzo.paulatto at cnrs.fr> 寫道：
>
>
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> On 15/11/2023 08:45, ludwigboltzmann.sc10 at nycu.edu.tw 
> <mailto:ludwigboltzmann.sc10 at nycu.edu.tw> wrote:
>  Hope this will give you an idea why such*symmetry reduced* supercell 
> calculation is being carried out.
>
>
> The idea is "symmetry reduced".
>
> I.e. DFPT allows you to simulate a monochromatic perturbation of any 
> periodicity with a unit cell, while frozen phonon can only simulate 
> perturbation which have the periodicity of the cell you are using. But 
> to break symmetry, you need a supercell.
>
> kind regards
>
> L
>
>
> _______________________________________________
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing listusers at lists.quantum-espresso.org
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-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
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