program print_ij_H_kl_det_v0

  implicit none

  BEGIN_DOC
  ! perturbative approach to build psi_postsvd
  ! without OMP
  END_DOC

  read_wf = .True.
  TOUCH read_wf

  PROVIDE N_int

  call run()
end


subroutine run

  USE OMP_LIB

  implicit none

  integer(bit_kind)             :: det1(N_int,2), det2(N_int,2)
  integer                       :: degree, i_state
  double precision              :: h12

  integer(kind=8)               :: W_tbeg, W_tend, W_tbeg_it, W_tend_it, W_ir
  real(kind=8)                  :: W_tot_time
  integer                       :: nb_taches

  !$OMP PARALLEL
  nb_taches = OMP_GET_NUM_THREADS()
  !$OMP END PARALLEL

  call SYSTEM_CLOCK(COUNT=W_tbeg, COUNT_RATE=W_ir)

  i_state = 1

  det1(:,1) = psi_det_alpha_unique(:,1)
  det2(:,1) = psi_det_alpha_unique(:,1)
  det1(:,2) = psi_det_beta_unique(:,1)
  det2(:,2) = psi_det_beta_unique(:,1)
  call get_excitation_degree_spin(det1(1,1),det2(1,1),degree,N_int)
  call get_excitation_degree(det1,det2,degree,N_int)
  call i_H_j(det1, det2, N_int, h12)

  ! ---------------------------------------------------------------------------------------
  !                      construct the initial CI matrix

  print *, ' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~'
  print *, ' CISD matrix:', n_det_alpha_unique,'x',n_det_beta_unique
  print *, ' N det      :', N_det
  print *, ' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~'

  ! ---------------------------------------------------------------------------------------

  ! get < d_i d_j | H | d_k d_l > in det basis
  call const_ij_H_kl_det()


  call SYSTEM_CLOCK(COUNT=W_tend, COUNT_RATE=W_ir)
  W_tot_time   = real(W_tend - W_tbeg, kind=8) / real(W_ir, kind=8)
  print *, ' ___________________________________________________________________'
  print *, ' '
  !print *, " Execution avec ", nb_taches, "threads"
  print *, " Execution avec 1 threads"
  print *, " total elapsed time (min) = ", W_tot_time/60.d0
  print *, ' ___________________________________________________________________'

end





subroutine const_ij_H_kl_det()

  implicit none

  integer(bit_kind)             :: det1(N_int,2)
  integer(bit_kind)             :: det2(N_int,2)
  integer                       :: degree, na, nb

  integer                       :: i, j, k, l
  double precision              :: h12

  double precision              :: nb_nz, n_tot

  double precision              :: t1, t2

  print *, ""
  print *, " start const_ij_H_kl_det"
  call wall_time(t1)

  na = n_det_alpha_unique
  nb = n_det_beta_unique

  nb_nz = 0.d0
  do l = 1, nb
    det2(:,2) = psi_det_beta_unique(:,l)
    do j = 1, nb
      det1(:,2) = psi_det_beta_unique(:,j)

      call get_excitation_degree_spin(det1(1,2),det2(1,2),degree,N_int)
      if(degree .gt. 2) cycle

      do k = 1, na
        det2(:,1) = psi_det_alpha_unique(:,k)
        do i = 1, na
          det1(:,1) = psi_det_alpha_unique(:,i)

          call get_excitation_degree(det1,det2,degree,N_int)
          if(degree .gt. 2) cycle

          call i_H_j(det1, det2, N_int, h12)
          if(dabs(h12).le.(1d-12)) cycle
          
          ! need nbs
          nb_nz = nb_nz + 1.d0
          write(7000, '(4(I8,2X),(F20.13))') i, j, k, l, h12
        enddo
      enddo
    enddo
  enddo

  n_tot = dble(na)**2 * dble(nb)**2

  print *, ' na                    = ', na
  print *, ' nb                    = ', nb

  print *, ' nb non zero elemenets = ', nb_nz
  print *, ' n_tot                 = ', n_tot

  print *, ' %  non zero elemenets = ', 100.d0 * dexp( dlog(nb_nz) - dlog(n_tot) )

  call wall_time(t2)
  print *, " end const_ij_H_kl_det after (min) ", (t2-t1)/60.
  print *, ""

  return
end subroutine const_ij_H_kl_det