[Pw_forum] fft 3D

[Andrei Malashevich]

Dear Paolo and Lorenzo,

Thank you for such a detailed explanation!

Best regards,
Andrei


On Fri, May 17, 2013 at 3:59 AM, Paolo Giannozzi
<paolo.giannozzi at uniud.it>wrote:

> On Fri, 2013-05-17 at 01:18 +0800, Fang Liu wrote:
>
> > That is, I use a 25*25*25 3D FFT mesh. The original 3d array g3d =
> > (1.0_DP, -1.0_DP).
> >
> > First I do invfft and then fwfft. Finally I don't get the original 3d
> > array, but a 3d array g3dnew in which
> > g3dnew(1),g3dnew(2),...,g3dnew(3025)= (1.0_DP, -1.0_DP)
> > while g3dnew(3026),....,g3dnew(15625)=0.0.
>
> > But if I do fwfft first and then invfft, I can finally get the
> > original 3d array.
>
> The 3D FFTs in QE are tailored for the "dual-space" algorithm
> as used in plane-wave codes. If you specify "Wave", the code
> will assume that there are no G components beyond a sphere
> of radius G^2 < Ecut. For instance, when performing the G->R
> FT along z, many vectors will be ignored, because they are
> assumed to contain zeros. See Lorenzo's post for a more detailed
> explanation.
>
> P.
>
> --
>  Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
>  Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>  Phone +39-0432-558216, fax +39-0432-558222
>
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