[Pw_forum] fft 3D

[Paolo Giannozzi]

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