[Pw_forum] fft 3D

[Fang Liu]

Dear Paolo,
Dear Lorenzo,

Thank you very much for your detailed explanation.
Now I understand fft in QE.

My another question related to this fft problem is in
the vloc_psi_gamma subroutine.

In this subroutine, the input Vloc is a real vector in 3D fft real space.
Psi is real in real space, so after we do

        DO j = 1, dffts%nnr
           psic (j) = psic (j) * v(j)
        ENDDO

psic is still real in real space. Then by
        CALL fwfft ('Wave', psic, dffts)
we get psic in G space which satisfies psic*(G)=psic(-G).
So in QE by
           DO j = 1, n
              hpsi (j, ibnd)   = hpsi (j, ibnd)   + psic (nls(igk(j)))
           ENDDO
only psic on the points G=0 and G>0 are stored.

But if I want to revise this subroutine by letting Vloc is a general
complex vector,
then after

        DO j = 1, dffts%nnr
           psic (j) = psic (j) * v(j)
        ENDDO

        CALL fwfft ('Wave', psic, dffts)
we get psic in G space which doesn't satisfy psic*(G)=psic(-G) any more.
How can I get the correct order for psic on all of the points G=0 and G>0
and G<0
so that it can be consistent with H*psic?

Thank you.

Best,
Fang






2013/5/17 Paolo Giannozzi <paolo.giannozzi at uniud.it>

> On Fri, 2013-05-17 at 11:46 +0200, Gabriele Sclauzero wrote:
> >
> > >> In real space, the wavefunction is a real vector.
> > >> only for k=0 or if there is inversion symmetry
> >
> > Isn't it enough to have time reversal symmetry?
>
> time-reversal symmetry guarantees that \psi^*_{-k}=\psi_{k},
> IIRC, so it guarantees that wavefunctions are real at k=0.
> At a general k, wavefunctions are obviously complex,
> but in presence of inversion symmetry one can always
> write them as a purely real or purely imaginary function
> times the Bloch factor. This is however not currently
> used in QE.
>
> 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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