[Q-e-developers] Fast Fourier Transform

[Stefano de Gironcoli]

It's confusing...

for a given grid there are two type of FFTs

the one used to transform wavefunctions, limited to G-vectors up to Ecutwfc

and the one associated to the density/potential, up to 4*Ecutwfc or Ecutrho

There are three grids. the Smooth one (dffts: accommodating G-vectors up 
to 4*Ecutwfc), the Dense one (dfftp: accommodating G-vectors up to 
Ecutrho) and the Custom one (used in exx calculations), for a total of 
2x3 = 6 combinations.

only five are actually used:

Wave: transforms wavefunction on the smooth grid (dffts)

Smooth: transforms densities/potentials on the smooth grid (dffts)

Dense: transforms densities/potentials on the dense grid (dfftp)

CustomWave: transforms wavefunction on the custom grid

Custom: transforms densities/potentials on the custom grid.

The missing combination (wavefunction on the dense grid) is never needed.

It would make sense to redefine the type of fft as 'Rho/Wave' but for 
historical and timing-definition reasons this is not done (yet?).

Have a look to FFTXlib/fft_fwinv.f90 if you want to see how things work 
in detail.

... and actually there are three types of FFTs: Rho, Wave, and Wave with 
task groups, the last one only used on the smooth grid as 
invfft/fwfft('tgWave', psic, dffts)

stefano


On 06/11/2017 14:01, Miguel Carvajal wrote:
> Dear all,
> can some one explain to me what is the difference of calling
>
>
>
> CALL fwfft ('Custom', rhoc(:,ii), dfftt)
> CALL fwfft (‘Wave', rhoc(:,ii), dfftt)
> CALL fwfft (‘Dense', rhoc(:,ii),dfftt)
> CALL fwfft (’Smooth', rhoc(:,ii), dfftt)
> when doing FFT on QE; as well as the appropriate grid to be used in 
> each instance: *dfft, dfftp or  dffts*.
> I have some ideas but not a full understanding of it.
> Best regards,
>
> Lic. Miguel Carvajal - Instituto Balseiro - Bariloche
> National Atomic Energy Comission
> [+54] 2944782429 (tel) / krvajal.miguel (skype)
> LinkedIn: in/krvajal <https://www.linkedin.com/in/krvajal/>
>
>
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