[Pw_forum] Supercell size in phonon calculations

[stefano de gironcoli]

On 06/09/2017 11:22, Charlie Ruffman wrote:
>
> Hi all,
>
>
> I have recently become interested in how the size of the unit cell in 
> phonon calculations effects the results produced by the calculation 
> (Raman spectra in particular). It seems that increasing the number of 
> atoms explicitly simulated should also increase the number of normal 
> modes calcuated?
>
>
It does. these are hovewer modes that would correspond to a q/=0 in the 
original primitive cell... as such they should be raman-inactive.
The calculation in the supercell, if properly done (same cutoff, 
equivalent k-points, sufficient scf convergence) should be exactly 
comparable to the superposition of the results in the primitive cell for 
the q-vectors that map into the Gamma point of the supercell.

stefano

> Does anyone know if using the primitive cell will produce more or less 
> accurate results than a larger supercell?
>
>
> Thanks
>
>
> -- 
> Charlie Ruffman | BSc
> BSc(Hons) Student
>
> Chemistry Department
> University of Otago
> Tel 64 3 479 7930
> PO Box 56, Dunedin 9054
> New Zealand
>
> Website _http://neon.otago.ac.nz/research/alg/ 
> <http://neon.otago.ac.nz/research/alg/>_
>
>
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