[Q-e-developers] Fast Fourier Transform

[Miguel Carvajal]

I seems that in the current codebase some renaming have been made to the
fft grids.
Now there exist the `Rho` grid and the `Wave` grid. Can some one explain
the mappings from this new name to the old ones and what happened to the
`CustomWave` grid.
Best regards,




*Miguel Carvajal*   PhD
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On Thu, Nov 9, 2017 at 6:51 AM, Carlo Cavazzoni <c.cavazzoni at cineca.it>
wrote:

> It may help thinking that we use fft to compute a non linear
> Operator (convolution), then the result is represented on
> A denser grid in order not to loose accuracy.
> So we end up with 2 fft grids, for each type,
> Whereas different types are needed to cope with different observables
> With different basis sets.
> In principle we can do everything using just one grid, the largest,
> But we would waste a lot of space and cycles
>
> Carlo
> ----- Stefano de Gironcoli <degironc at sissa.it> wrote:
> > 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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> >
>
>
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