[Pw_forum] phonon calculation

[Lorenzo Paulatto]

Dear Suchitra,
as long as you are at Gamma, there is a chance that a finite difference 
(a.k.a. frozen phonons) calculation be faster, or at least easier to run 
in smaller chunks, than linear response. But I really see no way around 
the X point, it will require a supercell.

I honestly think that your best bet would be to use some localized basis 
set code, it is handy to have some plane wave results to compare, as it 
can be quite difficult to ensure that the localized basis sets are 
converged.

Carbon nanotubes are very well tested, and there is plenty of literature 
on the subject, e.g. from the open source code CP2K. See Phys. Rev. 
Lett. 103, 096102.

That said, an analysis of anharmonicity in perturbation theory, as done 
by the d3q-thermal2 codes, as well as the competing codes phono3py or 
ShengBTE, is probably prohibitive, even if based on a fast localized 
basis set code. For example, the 3-body force constant matrices with 64 
atoms, for a 2x2x1 grid, would take 8*(2*2*1*3*64)^3 bytes, i.e. more 
than 3.5 GB of memory, which is unmanageable. Sticking with the Gamma 
point for the anharmonic part is probably required.

Keep in mind that Fourier interpolation will still allow you to get some 
dispersion, and that the fine structure of phonon linewidths comes from 
the joint density of states, more than the actual values of the 3rd 
order dynamical "matrices".


On 26/09/17 07:14, Suchitra Kumari wrote:
> Dear all,
>                       I am looking for the phonon calculation of silicon 
> nanowire of 62 atoms and d3q calculation for the same but i got stuck in 
> the phonon calculation as the structure is having too many atoms and it 
> will take too much time.
>                      So, is this enough to do the calculation at gamma 
> and x-point q-points??will it give the optimized result for phonon and 
> d3q calculation??or is there any other means for phonon and d3q 
> calculation of large structure.
> 
> 
> Regards,
> suchitra
> 
> 
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-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 442 79822 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
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