SUBROUTINE fft6_g2r ( f_g, f_r)

  USE gsmooth, ONLY : nrxxs, nr1s, nr2s, nr3s, nrx1s, nrx2s, nrx3s, nls, ngms, nrxxs
  USE kinds, ONLY : DP 
  USE cell_base, ONLY : omega, alat

  implicit none
  integer :: ig, igp, ir, irp
  complex(DP) :: f_r (nrxxs,nrxxs), f_g(ngms,ngms), aux(nrxxs)
  complex(DP) :: czero

  
  czero = (1.0d0, 0.0d0)

  !
  do ig = 1, ngms
    aux = czero
    do igp = 1, ngms
      aux(nls(igp)) = f_g(ig,igp)
    enddo
    call cft3s ( aux, nr1s, nr2s, nr3s,  1)
    f_r(ig,1:nrxxs) = aux / omega
  enddo
  !
  ! the conjg/conjg is to calculate sum_G f(G) exp(-iGr)
  ! following teh convention set in the paper
  ! [because the standard transform is sum_G f(G) exp(iGr) ]
  !
  do irp = 1, nrxxs
    aux = czero
    do ig = 1, ngms
      aux(nls(ig)) = conjg ( f_r(ig,irp) )
    enddo
    call cft3s ( aux, nr1s, nr2s, nr3s,  1)
    f_r(1:nrxxs,irp) = conjg ( aux )
  enddo
  !
END SUBROUTINE fft6_g2r