!
! Copyright (C) 2001 PWSCF group
! This file is distributed under the terms of the
! GNU General Public License. See the file `License'
! in the root directory of the present distribution,
! or http://www.gnu.org/copyleft/gpl.txt .
!
#include "f_defs.h"
!
!----------------------------------------------------------------------
subroutine gen_us_dj (ik, dvkb)
  !----------------------------------------------------------------------
  !
  !   Calculates the beta function pseudopotentials with
  !   the derivative of the Bessel functions
  !
  USE kinds,      ONLY : DP
  USE parameters, ONLY : nbrx
  USE constants,  ONLY : tpi
  USE ions_base,  ONLY : nat, ntyp => nsp, ityp, tau
  USE cell_base,  ONLY : tpiba
  USE klist,      ONLY : xk
  USE gvect,      ONLY : ig1, ig2, ig3, eigts1, eigts2, eigts3, g
  USE wvfct,      ONLY : npw, npwx, igk
  USE uspp,       ONLY : nkb, indv, nhtol, nhtolm
  USE us,         ONLY : tab, dq
  USE uspp_param, ONLY : lmaxkb, nbeta, nh
  !
  implicit none
  !
  integer :: ik
  complex(kind=DP) :: dvkb (npwx, nkb)
  !
  ! local variables
  !
  integer :: ikb, nb, ih, ig, i0, i1, i2, i3 , nt
  ! counter on beta functions
  ! counter on beta functions
  ! counter on beta functions
  ! counter on G vectors
  ! index of the first nonzero point in the r
  ! counter on atomic type

  real(kind=DP) :: arg, px, ux, vx, wx
  ! argument of the atomic phase factor

  complex(kind=DP) :: phase, pref
  ! atomic phase factor
  ! prefactor

  integer :: na, i, l, iig, lm
  real(kind=DP), allocatable :: djl (:,:,:), ylm (:,:), q (:), gk (:,:)
  real(kind=DP) ::  qt, dv, eps
  parameter (eps = 1.0e-8)

  complex(kind=DP), allocatable :: sk (:)

  call start_clock('stres_us31')

  if (nkb.eq.0) return
  allocate (djl( npw , nbrx , ntyp))    
  allocate (ylm( npw ,(lmaxkb + 1) **2))    
  allocate (gk( 3, npw))    
  allocate (q( npw))    
  do ig = 1, npw
     gk (1,ig) = xk (1, ik) + g(1, igk(ig) )
     gk (2,ig) = xk (2, ik) + g(2, igk(ig) )
     gk (3,ig) = xk (3, ik) + g(3, igk(ig) )
     q (ig) = gk(1, ig)**2 +  gk(2, ig)**2 + gk(3, ig)**2
  enddo

  call stop_clock('stres_us31')
  call start_clock('stres_us32')
  call ylmr2 ((lmaxkb+1)**2, npw, gk, q, ylm)
  call stop_clock('stres_us32')
  call start_clock('stres_us33')

  do nt = 1, ntyp
     do nb = 1, nbeta (nt)
        do ig = 1, npw
           qt = sqrt(q (ig)) * tpiba
           px = qt / dq - int (qt / dq)
           ux = 1.d0 - px
           vx = 2.d0 - px
           wx = 3.d0 - px
           i0 = qt / dq + 1
           i1 = i0 + 1
           i2 = i0 + 2
           i3 = i0 + 3
           djl(ig,nb,nt) = ( tab (i0, nb, nt) * (-vx*wx-ux*wx-ux*vx)/6.d0 + &
                             tab (i1, nb, nt) * (+vx*wx-px*wx-px*vx)/2.d0 - &
                             tab (i2, nb, nt) * (+ux*wx-px*wx-px*ux)/2.d0 + &
                             tab (i3, nb, nt) * (+ux*vx-px*vx-px*ux)/6.d0 )/dq
        enddo
     enddo
  enddo
  call stop_clock('stres_us33')
  call start_clock('stres_us34')

  deallocate (q)
  deallocate (gk)

  allocate (sk( npw))    
  ikb = 0
  do nt = 1, ntyp
     do na = 1, nat
        if (ityp (na) .eq.nt) then
           arg = (xk (1, ik) * tau(1,na) + &
                  xk (2, ik) * tau(2,na) + &
                  xk (3, ik) * tau(3,na) ) * tpi
           phase = DCMPLX (cos (arg), - sin (arg) )
           do ig = 1, npw
              iig = igk (ig)
              sk (ig) = eigts1 (ig1 (iig), na) * &
                        eigts2 (ig2 (iig), na) * &
                        eigts3 (ig3 (iig), na) * phase
           enddo
           do ih = 1, nh (nt)
              nb = indv (ih, nt)
              l = nhtol (ih, nt)
              lm= nhtolm(ih, nt)
              ikb = ikb + 1
              pref = (0.d0, -1.d0) **l
              !
              do ig = 1, npw
                 dvkb (ig, ikb) = djl (ig, nb, nt) * sk (ig) * ylm (ig, lm) &
                      * pref
              enddo
           enddo
        endif
     enddo

  enddo
  call stop_clock('stres_us34')

  if (ikb.ne.nkb) call errore ('gen_us_dj', 'unexpected error', 1)
  deallocate (sk)
  deallocate (ylm)
  deallocate (djl)
  return
end subroutine gen_us_dj