!--------------------------------------------------------------------------------------------------!
!   CP2K: A general program to perform molecular dynamics simulations                              !
!   Copyright 2000-2025 CP2K developers group <https://cp2k.org>                                   !
!                                                                                                  !
!   SPDX-License-Identifier: GPL-2.0-or-later                                                      !
!--------------------------------------------------------------------------------------------------!

! **************************************************************************************************
!> \brief All the kernel specific subroutines for XAS TDP calculations
!> \author A. Bussy (03.2019)
! **************************************************************************************************

MODULE xas_tdp_kernel
   USE basis_set_types,                 ONLY: gto_basis_set_p_type
   USE cp_dbcsr_api,                    ONLY: &
        dbcsr_add, dbcsr_complete_redistribute, dbcsr_copy, dbcsr_create, dbcsr_desymmetrize, &
        dbcsr_distribution_get, dbcsr_distribution_new, dbcsr_distribution_release, &
        dbcsr_distribution_type, dbcsr_filter, dbcsr_finalize, dbcsr_get_block_p, dbcsr_get_info, &
        dbcsr_get_stored_coordinates, dbcsr_iterator_blocks_left, dbcsr_iterator_next_block, &
        dbcsr_iterator_start, dbcsr_iterator_stop, dbcsr_iterator_type, dbcsr_multiply, &
        dbcsr_p_type, dbcsr_put_block, dbcsr_release, dbcsr_reserve_block2d, dbcsr_set, &
        dbcsr_transposed, dbcsr_type, dbcsr_type_no_symmetry, dbcsr_type_symmetric
   USE cp_dbcsr_operations,             ONLY: cp_dbcsr_dist2d_to_dist,&
                                              dbcsr_deallocate_matrix_set
   USE dbt_api,                         ONLY: dbt_get_block,&
                                              dbt_iterator_blocks_left,&
                                              dbt_iterator_next_block,&
                                              dbt_iterator_start,&
                                              dbt_iterator_stop,&
                                              dbt_iterator_type,&
                                              dbt_type
   USE distribution_2d_types,           ONLY: distribution_2d_type
   USE kinds,                           ONLY: dp
   USE message_passing,                 ONLY: mp_para_env_type
   USE particle_methods,                ONLY: get_particle_set
   USE particle_types,                  ONLY: particle_type
   USE qs_environment_types,            ONLY: get_qs_env,&
                                              qs_environment_type
   USE qs_integral_utils,               ONLY: basis_set_list_setup
   USE qs_kind_types,                   ONLY: qs_kind_type
   USE util,                            ONLY: get_limit
   USE xas_tdp_types,                   ONLY: donor_state_type,&
                                              get_proc_batch_sizes,&
                                              xas_tdp_control_type,&
                                              xas_tdp_env_type

!$ USE OMP_LIB, ONLY: omp_get_max_threads, omp_get_thread_num
#include "./base/base_uses.f90"

   IMPLICIT NONE
   PRIVATE

   CHARACTER(len=*), PARAMETER, PRIVATE :: moduleN = 'xas_tdp_kernel'

   PUBLIC :: kernel_coulomb_xc, kernel_exchange, contract2_AO_to_doMO, &
             reserve_contraction_blocks, ri_all_blocks_mm

CONTAINS

! **************************************************************************************************
!> \brief Computes, if asked for it, the Coulomb and XC kernel matrices, in the usuall matrix format
!> \param coul_ker pointer the the Coulomb kernel matrix (can be void pointer)
!> \param xc_ker array of pointer to the different xc kernels (5 of them):
!>               1) the restricted closed-shell singlet kernel
!>               2) the restricted closed-shell triplet kernel
!>               3) the spin-conserving open-shell xc kernel
!>               4) the on-diagonal spin-flip open-shell xc kernel
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
!> \note Coulomb and xc kernel are put together in the same routine because they use the same RI
!>       Coulomb: (aI|Jb) = (aI|P) (P|Q)^-1 (Q|Jb)
!>       XC : (aI|fxc|Jb) = (aI|P) (P|Q)^-1 (Q|fxc|R) (R|S)^-1 (S|Jb)
!>       In the above formula, a,b label the sgfs
!>       The routine analyses the xas_tdp_control to know which kernel must be computed and how
!>       (open-shell, singlet, triplet, ROKS, LSD, etc...)
!>       On entry, the pointers should be allocated
! **************************************************************************************************
   SUBROUTINE kernel_coulomb_xc(coul_ker, xc_ker, donor_state, xas_tdp_env, xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: coul_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: xc_ker
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      CHARACTER(len=*), PARAMETER                        :: routineN = 'kernel_coulomb_xc'

      INTEGER                                            :: batch_size, bo(2), handle, i, ibatch, &
                                                            iex, lb, natom, nbatch, ndo_mo, &
                                                            ndo_so, nex_atom, nsgfp, ri_atom, &
                                                            source, ub
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      LOGICAL                                            :: do_coulomb, do_sc, do_sf, do_sg, do_tp, &
                                                            do_xc, found
      REAL(dp), DIMENSION(:, :), POINTER                 :: PQ
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int
      TYPE(mp_para_env_type), POINTER                    :: para_env

      NULLIFY (contr1_int, PQ, para_env, dist, blk_size)

!  Initialization
      ndo_mo = donor_state%ndo_mo
      do_xc = xas_tdp_control%do_xc
      do_sg = xas_tdp_control%do_singlet
      do_tp = xas_tdp_control%do_triplet
      do_sc = xas_tdp_control%do_spin_cons
      do_sf = xas_tdp_control%do_spin_flip
      ndo_so = ndo_mo; IF (xas_tdp_control%do_uks) ndo_so = 2*ndo_mo
      ri_atom = donor_state%at_index
      CALL get_qs_env(qs_env, natom=natom, para_env=para_env)
      do_coulomb = xas_tdp_control%do_coulomb
      dist => donor_state%dbcsr_dist
      blk_size => donor_state%blk_size

!  If no Coulomb nor xc, simply exit
      IF ((.NOT. do_coulomb) .AND. (.NOT. do_xc)) RETURN

      CALL timeset(routineN, handle)

!  Contract the RI 3-center integrals once to get (aI|P)
      CALL contract2_AO_to_doMO(contr1_int, "COULOMB", donor_state, xas_tdp_env, xas_tdp_control, qs_env)

!  Deal with the Coulomb case
      IF (do_coulomb) CALL coulomb(coul_ker, contr1_int, dist, blk_size, xas_tdp_env, &
                                   xas_tdp_control, qs_env)

!  Deal with the XC case
      IF (do_xc) THEN

         ! In the end, we compute: (aI|fxc|Jb) = (aI|P) (P|Q)^-1 (Q|fxc|R) (R|S)^-1 (S|Jb)
         ! where fxc can take different spin contributions.

         ! Precompute the product (aI|P) * (P|Q)^-1 and store it in contr1_int
         PQ => xas_tdp_env%ri_inv_coul
         CALL ri_all_blocks_mm(contr1_int, PQ)

         ! If not already done (e.g. when multpile donor states for a given excited atom), broadcast
         ! the RI matrix (Q|fxc|R) on all procs
         IF (.NOT. xas_tdp_env%fxc_avail) THEN
            ! Find on which processor the integrals (Q|fxc|R) for this atom are stored
            nsgfp = SIZE(PQ, 1)
            CALL get_qs_env(qs_env, para_env=para_env)
            found = .FALSE.
            nex_atom = SIZE(xas_tdp_env%ex_atom_indices)
            CALL get_proc_batch_sizes(batch_size, nbatch, nex_atom, para_env%num_pe)

            DO ibatch = 0, nbatch - 1

               bo = get_limit(nex_atom, nbatch, ibatch)
               DO iex = bo(1), bo(2)

                  IF (xas_tdp_env%ex_atom_indices(iex) == ri_atom) THEN
                     source = ibatch*batch_size
                     found = .TRUE. !but simply take the first
                     EXIT
                  END IF
               END DO !iex
               IF (found) EXIT
            END DO !ip

            ! Broadcast the integrals to all procs (deleted after all donor states for this atoms are treated)
            lb = 1; IF (do_sf .AND. .NOT. do_sc) lb = 4
            ub = 2; IF (do_sc) ub = 3; IF (do_sf) ub = 4
            DO i = lb, ub
               IF (.NOT. ASSOCIATED(xas_tdp_env%ri_fxc(ri_atom, i)%array)) THEN
                  ALLOCATE (xas_tdp_env%ri_fxc(ri_atom, i)%array(nsgfp, nsgfp))
               END IF
               CALL para_env%bcast(xas_tdp_env%ri_fxc(ri_atom, i)%array, source)
            END DO

            xas_tdp_env%fxc_avail = .TRUE.
         END IF

         ! Case study on the calculation type
         IF (do_sg .OR. do_tp) THEN
            CALL rcs_xc(xc_ker(1)%matrix, xc_ker(2)%matrix, contr1_int, dist, blk_size, &
                        donor_state, xas_tdp_env, xas_tdp_control, qs_env)
         END IF

         IF (do_sc) THEN
            CALL sc_os_xc(xc_ker(3)%matrix, contr1_int, dist, blk_size, donor_state, &
                          xas_tdp_env, xas_tdp_control, qs_env)
         END IF

         IF (do_sf) THEN
            CALL ondiag_sf_os_xc(xc_ker(4)%matrix, contr1_int, dist, blk_size, donor_state, &
                                 xas_tdp_env, xas_tdp_control, qs_env)
         END IF

      END IF ! do_xc

!  Clean-up
      CALL dbcsr_deallocate_matrix_set(contr1_int)

      CALL timestop(handle)

   END SUBROUTINE kernel_coulomb_xc

! **************************************************************************************************
!> \brief Create the matrix containing the Coulomb kernel, which is:
!>        (aI_sigma|J_tau b) ~= (aI_sigma|P) * (P|Q) * (Q|J_tau b)
!> \param coul_ker the Coulomb kernel
!> \param contr1_int the once contracted RI integrals (aI|P)
!> \param dist the inherited dbcsr ditribution
!> \param blk_size the inherited block sizes
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
! **************************************************************************************************
   SUBROUTINE coulomb(coul_ker, contr1_int, dist, blk_size, xas_tdp_env, xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: coul_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      LOGICAL                                            :: quadrants(3)
      REAL(dp), DIMENSION(:, :), POINTER                 :: PQ
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: lhs_int, rhs_int
      TYPE(dbcsr_type)                                   :: work_mat

      NULLIFY (PQ, rhs_int, lhs_int)

      ! Get the inver RI coulomb
      PQ => xas_tdp_env%ri_inv_coul

      ! Create a normal type work matrix
      CALL dbcsr_create(work_mat, name="WORK", matrix_type=dbcsr_type_no_symmetry, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)

      ! Compute the product (aI|P) * (P|Q)^-1 * (Q|Jb) = (aI|Jb)
      rhs_int => contr1_int ! the incoming contr1_int is not modified
      ALLOCATE (lhs_int(SIZE(contr1_int)))
      CALL copy_ri_contr_int(lhs_int, rhs_int) ! RHS containts (Q|JB)^T
      CALL ri_all_blocks_mm(lhs_int, PQ) ! LHS contatins (aI|P)*(P|Q)^-1

      !In the special case of ROKS, same MOs for each spin => put same (aI|Jb) product on the
      !alpha-alpha, alpha-beta and beta-beta quadrants of the kernel matrix.
      IF (xas_tdp_control%do_roks) THEN
         quadrants = [.TRUE., .TRUE., .TRUE.]
      ELSE
         quadrants = [.TRUE., .FALSE., .FALSE.]
      END IF
      CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                          eps_filter=xas_tdp_control%eps_filter)
      CALL dbcsr_finalize(work_mat)

      !Create the symmetric kernel matrix and redistribute work_mat into it
      CALL dbcsr_create(coul_ker, name="COULOMB KERNEL", matrix_type=dbcsr_type_symmetric, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)
      CALL dbcsr_complete_redistribute(work_mat, coul_ker)

      !clean-up
      CALL dbcsr_release(work_mat)
      CALL dbcsr_deallocate_matrix_set(lhs_int)

   END SUBROUTINE coulomb

! **************************************************************************************************
!> \brief Create the matrix containing the XC kenrel in the spin-conserving open-shell case:
!>        (aI_sigma|fxc|J_tau b) ~= (aI_sigma|P) (P|Q)^-1 (Q|fxc|R) (R|S)^-1 (S|J_tau b)
!> \param xc_ker the kernel matrix
!> \param contr1_int_PQ the once contracted RI integrals, with inverse coulomb: (aI_sigma|P) (P|Q)^-1
!> \param dist inherited dbcsr dist
!> \param blk_size inherited block sizes
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
!> note Prior to calling this function, the (Q|fxc|R) integral must be brodcasted to all procs
! **************************************************************************************************
   SUBROUTINE sc_os_xc(xc_ker, contr1_int_PQ, dist, blk_size, donor_state, xas_tdp_env, &
                       xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: xc_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int_PQ
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      INTEGER                                            :: ndo_mo, ri_atom
      LOGICAL                                            :: quadrants(3)
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: lhs_int, rhs_int
      TYPE(dbcsr_type)                                   :: work_mat

      NULLIFY (lhs_int, rhs_int)

      ! Initialization
      ndo_mo = donor_state%ndo_mo
      ri_atom = donor_state%at_index
      !normal type work matrix such that distribution of all spin quadrants match
      CALL dbcsr_create(work_mat, name="WORK", matrix_type=dbcsr_type_no_symmetry, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)

      rhs_int => contr1_int_PQ ! contains [ (aI|P)*(P|Q)^-1 ]^T
      ALLOCATE (lhs_int(SIZE(contr1_int_PQ))) ! will contain (aI|P)*(P|Q)^-1 * (Q|fxc|R)

      ! Case study: UKS or ROKS ?
      IF (xas_tdp_control%do_uks) THEN

         ! In the case of UKS, donor MOs might be different for different spins. Moreover, the
         ! fxc itself might change since fxc = fxc_sigma,tau
         ! => Carfully treat each spin-quadrant separately

         ! alpha-alpha spin quadrant (upper-lefet)
         quadrants = [.TRUE., .FALSE., .FALSE.]

         ! Copy the alpha part into lhs_int, multiply by the alpha-alpha (Q|fxc|R) and then
         ! by the alpha part of rhs_int
         CALL copy_ri_contr_int(lhs_int(1:ndo_mo), rhs_int(1:ndo_mo))
         CALL ri_all_blocks_mm(lhs_int(1:ndo_mo), xas_tdp_env%ri_fxc(ri_atom, 1)%array)
         CALL ri_int_product(work_mat, lhs_int(1:ndo_mo), rhs_int(1:ndo_mo), quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)

         ! alpha-beta spin quadrant (upper-right)
         quadrants = [.FALSE., .TRUE., .FALSE.]

         !Copy the alpha part into LHS, multiply by the alpha-beta kernel and the beta part of RHS
         CALL copy_ri_contr_int(lhs_int(1:ndo_mo), rhs_int(1:ndo_mo))
         CALL ri_all_blocks_mm(lhs_int(1:ndo_mo), xas_tdp_env%ri_fxc(ri_atom, 2)%array)
         CALL ri_int_product(work_mat, lhs_int(1:ndo_mo), rhs_int(ndo_mo + 1:2*ndo_mo), &
                             quadrants, qs_env, eps_filter=xas_tdp_control%eps_filter)

         ! beta-beta spin quadrant (lower-right)
         quadrants = [.FALSE., .FALSE., .TRUE.]

         !Copy the beta part into LHS, multiply by the beta-beta kernel and the beta part of RHS
         CALL copy_ri_contr_int(lhs_int(ndo_mo + 1:2*ndo_mo), rhs_int(ndo_mo + 1:2*ndo_mo))
         CALL ri_all_blocks_mm(lhs_int(ndo_mo + 1:2*ndo_mo), xas_tdp_env%ri_fxc(ri_atom, 3)%array)
         CALL ri_int_product(work_mat, lhs_int(ndo_mo + 1:2*ndo_mo), rhs_int(ndo_mo + 1:2*ndo_mo), &
                             quadrants, qs_env, eps_filter=xas_tdp_control%eps_filter)

      ELSE IF (xas_tdp_control%do_roks) THEN

         ! In the case of ROKS, fxc = fxc_sigma,tau is different for each spin quadrant, but the
         ! donor MOs remain the same

         ! alpha-alpha kernel in the upper left quadrant
         quadrants = [.TRUE., .FALSE., .FALSE.]

         !Copy the LHS and multiply by alpha-alpha kernel
         CALL copy_ri_contr_int(lhs_int, rhs_int)
         CALL ri_all_blocks_mm(lhs_int, xas_tdp_env%ri_fxc(ri_atom, 1)%array)
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)

         ! alpha-beta kernel in the upper-right quadrant
         quadrants = [.FALSE., .TRUE., .FALSE.]

         !Copy LHS and multiply by the alpha-beta kernel
         CALL copy_ri_contr_int(lhs_int, rhs_int)
         CALL ri_all_blocks_mm(lhs_int, xas_tdp_env%ri_fxc(ri_atom, 2)%array)
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)

         ! beta-beta kernel in the lower-right quadrant
         quadrants = [.FALSE., .FALSE., .TRUE.]

         !Copy the LHS and multiply by the beta-beta kernel
         CALL copy_ri_contr_int(lhs_int, rhs_int)
         CALL ri_all_blocks_mm(lhs_int, xas_tdp_env%ri_fxc(ri_atom, 3)%array)
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)

      END IF
      CALL dbcsr_finalize(work_mat)

      ! Create a symmetric kernel matrix and redistribute the normal work matrix into it
      CALL dbcsr_create(xc_ker, name="SC OS XC KERNEL", matrix_type=dbcsr_type_symmetric, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)
      CALL dbcsr_complete_redistribute(work_mat, xc_ker)

      !clean-up
      CALL dbcsr_deallocate_matrix_set(lhs_int)
      CALL dbcsr_release(work_mat)

   END SUBROUTINE sc_os_xc

! **************************************************************************************************
!> \brief Create the matrix containing the on-diagonal spin-flip XC kernel (open-shell), which is:
!>        (a I_sigma|fxc|J_tau b) * delta_sigma,tau, fxc = 1/(rhoa-rhob) * (dE/drhoa - dE/drhob)
!>        with RI: (a I_sigma|fxc|J_tau b) ~= (aI_sigma|P) (P|Q)^-1 (Q|fxc|R) (R|S)^-1 (S|J_tau b)
!> \param xc_ker the kernel matrix
!> \param contr1_int_PQ the once contracted RI integrals with coulomb product: (aI_sigma|P) (P|Q)^-1
!> \param dist inherited dbcsr dist
!> \param blk_size inherited block sizes
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
!> \note  It must be later on multiplied by the spin-swapped Q projector
!>        Prior to calling this function, the (Q|fxc|R) integral must be brodcasted to all procs
! **************************************************************************************************
   SUBROUTINE ondiag_sf_os_xc(xc_ker, contr1_int_PQ, dist, blk_size, donor_state, xas_tdp_env, &
                              xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: xc_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int_PQ
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      INTEGER                                            :: ndo_mo, ri_atom
      LOGICAL                                            :: quadrants(3)
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: lhs_int, rhs_int
      TYPE(dbcsr_type)                                   :: work_mat

      NULLIFY (lhs_int, rhs_int)

      ! Initialization
      ndo_mo = donor_state%ndo_mo
      ri_atom = donor_state%at_index
      !normal type work matrix such that distribution of all spin quadrants match
      CALL dbcsr_create(work_mat, name="WORK", matrix_type=dbcsr_type_no_symmetry, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)

      !Create a lhs_int, in which the whole (aI_sigma|P) (P|Q)^-1 (Q|fxc|R) will be put
      !because in spin-flip fxc is spin-independent, can take the product once and for all
      rhs_int => contr1_int_PQ
      ALLOCATE (lhs_int(SIZE(contr1_int_PQ)))
      CALL copy_ri_contr_int(lhs_int, rhs_int)
      CALL ri_all_blocks_mm(lhs_int, xas_tdp_env%ri_fxc(ri_atom, 4)%array)

      ! Case study: UKS or ROKS ?
      IF (xas_tdp_control%do_uks) THEN

         ! In the case of UKS, donor MOs might be different for different spins
         ! => Carfully treat each spin-quadrant separately
         ! NO alpha-beta because of the delta_sigma,tau

         ! alpha-alpha spin quadrant (upper-lefet)
         quadrants = [.TRUE., .FALSE., .FALSE.]
         CALL ri_int_product(work_mat, lhs_int(1:ndo_mo), rhs_int(1:ndo_mo), quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)

         ! beta-beta spin quadrant (lower-right)
         quadrants = [.FALSE., .FALSE., .TRUE.]
         CALL ri_int_product(work_mat, lhs_int(ndo_mo + 1:2*ndo_mo), rhs_int(ndo_mo + 1:2*ndo_mo), &
                             quadrants, qs_env, eps_filter=xas_tdp_control%eps_filter)

      ELSE IF (xas_tdp_control%do_roks) THEN

         ! In the case of ROKS, same donor MOs for both spins => can do it all at once
         ! But NOT the alpha-beta quadrant because of delta_sigma,tau

         quadrants = [.TRUE., .FALSE., .TRUE.]
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)

      END IF
      CALL dbcsr_finalize(work_mat)

      ! Create a symmetric kernel matrix and redistribute the normal work matrix into it
      CALL dbcsr_create(xc_ker, name="ON-DIAG SF OS XC KERNEL", matrix_type=dbcsr_type_symmetric, &
                        dist=dist, row_blk_size=blk_size, col_blk_size=blk_size)
      CALL dbcsr_complete_redistribute(work_mat, xc_ker)

      !clean-up
      CALL dbcsr_deallocate_matrix_set(lhs_int)
      CALL dbcsr_release(work_mat)

   END SUBROUTINE ondiag_sf_os_xc

! **************************************************************************************************
!> \brief Create the matrix containing the XC kernel in the restricted closed-shell case, for
!>        singlets: (aI|fxc|Jb) = (aI|P) (P|Q)^-1 (Q|fxc_alp,alp+fxc_alp,bet|R) (R|S)^-1 (S|Jb)
!>        triplets: (aI|fxc|Jb) = (aI|P) (P|Q)^-1 (Q|fxc_alp,alp-fxc_alp,bet|R) (R|S)^-1 (S|Jb)
!> \param sg_xc_ker the singlet kernel matrix
!> \param tp_xc_ker the triplet kernel matrix
!> \param contr1_int_PQ the once contracted RI integrals including inverse 2-center Coulomb prodcut:
!>                      (aI|P)*(P|Q)^-1
!> \param dist inherited dbcsr dist
!> \param blk_size inherited block sizes
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
! **************************************************************************************************
   SUBROUTINE rcs_xc(sg_xc_ker, tp_xc_ker, contr1_int_PQ, dist, blk_size, donor_state, &
                     xas_tdp_env, xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: sg_xc_ker, tp_xc_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int_PQ
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      INTEGER                                            :: nsgfp, ri_atom
      LOGICAL                                            :: quadrants(3)
      REAL(dp), ALLOCATABLE, DIMENSION(:, :)             :: fxc
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: lhs_int, rhs_int
      TYPE(dbcsr_type)                                   :: work_mat

      NULLIFY (lhs_int, rhs_int)

      ! Initialization
      ri_atom = donor_state%at_index
      nsgfp = SIZE(xas_tdp_env%ri_fxc(ri_atom, 1)%array, 1)
      rhs_int => contr1_int_PQ ! RHS contains [ (aI|P)*(P|Q)^-1 ]^T
      ALLOCATE (lhs_int(SIZE(contr1_int_PQ))) ! LHS will contatin (aI|P)*(P|Q)^-1 * (Q|fxc|R)

      ! Work structures
      ALLOCATE (fxc(nsgfp, nsgfp))
      CALL dbcsr_create(work_mat, name="WORK", matrix_type=dbcsr_type_no_symmetry, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)

      ! Case study: singlet and/or triplet ?
      IF (xas_tdp_control%do_singlet) THEN

         ! Take the sum of fxc for alpha-alpha and alpha-beta
         CALL dcopy(nsgfp*nsgfp, xas_tdp_env%ri_fxc(ri_atom, 1)%array, 1, fxc, 1)
         CALL daxpy(nsgfp*nsgfp, 1.0_dp, xas_tdp_env%ri_fxc(ri_atom, 2)%array, 1, fxc, 1)

         ! Copy the fresh lhs_int = (aI|P) (P|Q)^-1 and multiply by (Q|fxc|R)
         CALL copy_ri_contr_int(lhs_int, rhs_int)
         CALL ri_all_blocks_mm(lhs_int, fxc)

         ! Compute the final LHS RHS product => spin-restricted, only upper-left quadrant
         quadrants = [.TRUE., .FALSE., .FALSE.]
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)
         CALL dbcsr_finalize(work_mat)

         !Create the symmetric kernel matrix and redistribute work_mat into it
         CALL dbcsr_create(sg_xc_ker, name="XC SINGLET KERNEL", matrix_type=dbcsr_type_symmetric, &
                           dist=dist, row_blk_size=blk_size, col_blk_size=blk_size)
         CALL dbcsr_complete_redistribute(work_mat, sg_xc_ker)

      END IF

      IF (xas_tdp_control%do_triplet) THEN

         ! Take the difference of fxc for alpha-alpha and alpha-beta
         CALL dcopy(nsgfp*nsgfp, xas_tdp_env%ri_fxc(ri_atom, 1)%array, 1, fxc, 1)
         CALL daxpy(nsgfp*nsgfp, -1.0_dp, xas_tdp_env%ri_fxc(ri_atom, 2)%array, 1, fxc, 1)

         ! Copy the fresh lhs_int = (aI|P) (P|Q)^-1 and multiply by (Q|fxc|R)
         CALL copy_ri_contr_int(lhs_int, rhs_int)
         CALL ri_all_blocks_mm(lhs_int, fxc)

         ! Compute the final LHS RHS product => spin-restricted, only upper-left quadrant
         quadrants = [.TRUE., .FALSE., .FALSE.]
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter)
         CALL dbcsr_finalize(work_mat)

         !Create the symmetric kernel matrix and redistribute work_mat into it
         CALL dbcsr_create(tp_xc_ker, name="XC TRIPLET KERNEL", matrix_type=dbcsr_type_symmetric, &
                           dist=dist, row_blk_size=blk_size, col_blk_size=blk_size)
         CALL dbcsr_complete_redistribute(work_mat, tp_xc_ker)

      END IF

      ! clean-up
      CALL dbcsr_deallocate_matrix_set(lhs_int)
      CALL dbcsr_release(work_mat)
      DEALLOCATE (fxc)

   END SUBROUTINE rcs_xc

! **************************************************************************************************
!> \brief Computes the exact exchange kernel matrix using RI. Returns an array of 2 matrices,
!>        which are:
!>        1) the on-diagonal kernel: (ab|I_sigma J_tau) * delta_sigma,tau
!>        2) the off-diagonal spin-conserving kernel: (aJ_sigma|I_tau b) * delta_sigma,tau
!>        An internal analysis determines which of the above are computed (can range from 0 to 2),
!> \param ex_ker ...
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
!> \note In the case of spin-conserving excitation, the kernel must later be multiplied by the
!>       usual Q projector. In the case of spin-flip, one needs to project the excitations coming
!>       from alpha donor MOs on the unoccupied beta MOs. This is done by multiplying by a Q
!>       projector where the alpha-alpha and beta-beta quadrants are swapped
!>       The ex_ker array should be allocated on entry (not the internals)
! **************************************************************************************************
   SUBROUTINE kernel_exchange(ex_ker, donor_state, xas_tdp_env, xas_tdp_control, qs_env)

      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: ex_ker
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      CHARACTER(len=*), PARAMETER                        :: routineN = 'kernel_exchange'

      INTEGER                                            :: handle
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      LOGICAL                                            :: do_off_sc
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int

      NULLIFY (contr1_int, dist, blk_size)

      !Don't do anything if no hfx
      IF (.NOT. xas_tdp_control%do_hfx) RETURN

      CALL timeset(routineN, handle)

      dist => donor_state%dbcsr_dist
      blk_size => donor_state%blk_size

      !compute the off-diag spin-conserving only if not TDA and anything that is spin-conserving
      do_off_sc = (.NOT. xas_tdp_control%tamm_dancoff) .AND. &
                  (xas_tdp_control%do_spin_cons .OR. xas_tdp_control%do_singlet .OR. xas_tdp_control%do_triplet)

      ! Need the once contracted integrals (aI|P)
      CALL contract2_AO_to_doMO(contr1_int, "EXCHANGE", donor_state, xas_tdp_env, xas_tdp_control, qs_env)

!  The on-diagonal exchange : (ab|P) * (P|Q)^-1 * (Q|I_sigma J_tau) * delta_sigma,tau
      CALL ondiag_ex(ex_ker(1)%matrix, contr1_int, dist, blk_size, donor_state, xas_tdp_env, &
                     xas_tdp_control, qs_env)

!  The off-diag spin-conserving case: (aJ_sigma|P) * (P|Q)^-1 * (Q|I_tau b) * delta_sigma,tau
      IF (do_off_sc) THEN
         CALL offdiag_ex_sc(ex_ker(2)%matrix, contr1_int, dist, blk_size, donor_state, &
                            xas_tdp_env, xas_tdp_control, qs_env)
      END IF

      !clean-up
      CALL dbcsr_deallocate_matrix_set(contr1_int)

      CALL timestop(handle)

   END SUBROUTINE kernel_exchange

! **************************************************************************************************
!> \brief Create the matrix containing the on-diagonal exact exchange kernel, which is:
!>        (ab|I_sigma J_tau) * delta_sigma,tau, where a,b are AOs, I_sigma and J_tau are the donor
!>        spin-orbitals. A RI is done: (ab|I_sigma J_tau) = (ab|P) * (P|Q)^-1 * (Q|I_sigma J_tau)
!> \param ondiag_ex_ker the on-diagonal exchange kernel in dbcsr format
!> \param contr1_int the already once-contracted RI 3-center integrals (aI_sigma|P)
!>        where each matrix of the array contains the contraction for the donor spin-orbital I_sigma
!> \param dist the inherited dbcsr distribution
!> \param blk_size the inherited dbcsr block sizes
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
!> \note In the presence of a RI metric, we have instead M^-1 * (P|Q) * M^-1
! **************************************************************************************************
   SUBROUTINE ondiag_ex(ondiag_ex_ker, contr1_int, dist, blk_size, donor_state, xas_tdp_env, &
                        xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: ondiag_ex_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      INTEGER                                            :: blk, group, iblk, iso, jblk, jso, nblk, &
                                                            ndo_mo, ndo_so, nsgfa, nsgfp, ri_atom, &
                                                            source
      INTEGER, DIMENSION(:), POINTER                     :: col_dist, col_dist_work, row_dist, &
                                                            row_dist_work
      INTEGER, DIMENSION(:, :), POINTER                  :: pgrid
      LOGICAL                                            :: do_roks, do_uks, found
      REAL(dp), ALLOCATABLE, DIMENSION(:, :)             :: coeffs, ri_coeffs
      REAL(dp), DIMENSION(:, :), POINTER                 :: aIQ, pblock, PQ
      TYPE(dbcsr_distribution_type)                      :: opt_dbcsr_dist, work_dbcsr_dist
      TYPE(dbcsr_iterator_type)                          :: iter
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: matrix_s
      TYPE(dbcsr_type)                                   :: abIJ, mats_desymm, work_mat
      TYPE(mp_para_env_type), POINTER                    :: para_env

      NULLIFY (para_env, matrix_s, pblock, aIQ, row_dist, col_dist, row_dist_work, col_dist_work, pgrid)

      ! We want to compute (ab|I_sigma J_tau) = (ab|P) * (P|Q)^-1 * (Q|I_sigma J_tau)
      ! Already have (cJ_tau|P)  stored in contr1_int. Need to further contract the
      ! AOs with the coeff of the I_alpha spin-orbital.

      ! Initialization
      ndo_mo = donor_state%ndo_mo
      ri_atom = donor_state%at_index
      do_roks = xas_tdp_control%do_roks
      do_uks = xas_tdp_control%do_uks
      ndo_so = ndo_mo; IF (do_uks) ndo_so = 2*ndo_mo !if not UKS, same donor MOs for both spins
      PQ => xas_tdp_env%ri_inv_ex

      CALL get_qs_env(qs_env, para_env=para_env, matrix_s=matrix_s, natom=nblk)
      nsgfp = SIZE(PQ, 1)
      nsgfa = SIZE(donor_state%contract_coeffs, 1)
      ALLOCATE (coeffs(nsgfp, ndo_so), ri_coeffs(nsgfp, ndo_so))

      ! a and b need to overlap for non-zero (ab|IJ) => same block structure as overlap S
      ! need compatible distribution_2d with 3c tensor + normal type
      CALL cp_dbcsr_dist2d_to_dist(xas_tdp_env%opt_dist2d_ex, opt_dbcsr_dist)

      CALL dbcsr_desymmetrize(matrix_s(1)%matrix, mats_desymm)

      CALL dbcsr_create(abIJ, template=mats_desymm, name="(ab|IJ)", dist=opt_dbcsr_dist)
      CALL dbcsr_complete_redistribute(mats_desymm, abIJ)

      CALL dbcsr_release(mats_desymm)

      ! Create a work distribution based on opt_dbcsr_dist, but for full size matrices
      CALL dbcsr_distribution_get(opt_dbcsr_dist, row_dist=row_dist, col_dist=col_dist, group=group, &
                                  pgrid=pgrid)

      ALLOCATE (row_dist_work(ndo_so*nblk))
      ALLOCATE (col_dist_work(ndo_so*nblk))
      DO iso = 1, ndo_so
         row_dist_work((iso - 1)*nblk + 1:iso*nblk) = row_dist(:)
         col_dist_work((iso - 1)*nblk + 1:iso*nblk) = col_dist(:)
      END DO

      CALL dbcsr_distribution_new(work_dbcsr_dist, group=group, pgrid=pgrid, row_dist=row_dist_work, &
                                  col_dist=col_dist_work)

      CALL dbcsr_create(work_mat, name="WORK", matrix_type=dbcsr_type_no_symmetry, dist=work_dbcsr_dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)

      ! Loop over donor spin-orbitals. End matrix is symmetric => span only upper half
      DO iso = 1, ndo_so

         ! take the (aI|Q) block in contr1_int that has a centered on the excited atom
         CALL dbcsr_get_stored_coordinates(contr1_int(iso)%matrix, ri_atom, ri_atom, source)
         IF (para_env%mepos == source) THEN
            CALL dbcsr_get_block_p(contr1_int(iso)%matrix, ri_atom, ri_atom, aIQ, found)
         ELSE
            ALLOCATE (aIQ(nsgfa, nsgfp))
         END IF
         CALL para_env%bcast(aIQ, source)

         ! get the contraction (Q|IJ) by taking (Q|Ia)*contract_coeffs and put it in coeffs
         CALL dgemm('T', 'N', nsgfp, ndo_so, nsgfa, 1.0_dp, aIQ, nsgfa, donor_state%contract_coeffs, &
                    nsgfa, 0.0_dp, coeffs, nsgfp)

         ! take (P|Q)^-1 * (Q|IJ) and put that in ri_coeffs
         CALL dgemm('N', 'N', nsgfp, ndo_so, nsgfp, 1.0_dp, PQ, nsgfp, coeffs, nsgfp, 0.0_dp, &
                    ri_coeffs, nsgfp)

         IF (.NOT. para_env%mepos == source) DEALLOCATE (aIQ)

         DO jso = iso, ndo_so

            ! There is no alpha-beta exchange. In case of UKS, iso,jso span all spin-orbitals
            ! => CYCLE if iso and jso are indexing MOs with different spin (and we have UKS)
            IF (do_uks .AND. (iso <= ndo_mo .AND. jso > ndo_mo)) CYCLE

            ! compute (ab|IJ) = sum_P (ab|P) * (P|Q)^-1 * (Q|IJ)
            CALL dbcsr_set(abIJ, 0.0_dp)
            CALL contract3_RI_to_doMOs(xas_tdp_env%ri_3c_ex, ri_coeffs(:, jso), abIJ, ri_atom)

            ! Loop over (ab|IJ) and copy into work. OK because dist are made to match
            CALL dbcsr_iterator_start(iter, abIJ)
            DO WHILE (dbcsr_iterator_blocks_left(iter))

               CALL dbcsr_iterator_next_block(iter, row=iblk, column=jblk, blk=blk)
               IF (iso == jso .AND. jblk < iblk) CYCLE

               CALL dbcsr_get_block_p(abIJ, iblk, jblk, pblock, found)

               IF (found) THEN
                  CALL dbcsr_put_block(work_mat, (iso - 1)*nblk + iblk, (jso - 1)*nblk + jblk, pblock)

                  !In case of ROKS, we have (ab|IJ) for alpha-alpha spin, but it is the same for
                  !beta-beta => replicate the blocks (alpha-beta is zero)
                  IF (do_roks) THEN
                     !the beta-beta block
                     CALL dbcsr_put_block(work_mat, (ndo_so + iso - 1)*nblk + iblk, &
                                          (ndo_so + jso - 1)*nblk + jblk, pblock)
                  END IF
               END IF

            END DO !iterator
            CALL dbcsr_iterator_stop(iter)

         END DO !jso
      END DO !iso

      CALL dbcsr_finalize(work_mat)
      CALL dbcsr_create(ondiag_ex_ker, name="ONDIAG EX KERNEL", matrix_type=dbcsr_type_symmetric, &
                        dist=dist, row_blk_size=blk_size, col_blk_size=blk_size)
      CALL dbcsr_complete_redistribute(work_mat, ondiag_ex_ker)

      !Clean-up
      CALL dbcsr_release(work_mat)
      CALL dbcsr_release(abIJ)
      CALL dbcsr_distribution_release(opt_dbcsr_dist)
      CALL dbcsr_distribution_release(work_dbcsr_dist)
      DEALLOCATE (col_dist_work, row_dist_work)

   END SUBROUTINE ondiag_ex

! **************************************************************************************************
!> \brief Create the matrix containing the off-diagonal exact exchange kernel in the spin-conserving
!>        case (which also includes excitations from the closed=shell ref state ) This matrix reads:
!>        (aJ_sigma|I_tau b) * delta_sigma,tau , where a, b are AOs and J_sigma, I_tau are the donor
!>        spin-orbital. A RI is done: (aJ_sigma|I_tau b) = (aJ_sigma|P) * (P|Q)^-1 * (Q|I_tau b)
!> \param offdiag_ex_ker the off-diagonal, spin-conserving exchange kernel in dbcsr format
!> \param contr1_int the once-contracted RI integrals: (aJ_sigma|P)
!> \param dist the inherited dbcsr ditribution
!> \param blk_size the inherited block sizes
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
! **************************************************************************************************
   SUBROUTINE offdiag_ex_sc(offdiag_ex_ker, contr1_int, dist, blk_size, donor_state, xas_tdp_env, &
                            xas_tdp_control, qs_env)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: offdiag_ex_ker
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr1_int
      TYPE(dbcsr_distribution_type), POINTER             :: dist
      INTEGER, DIMENSION(:), POINTER                     :: blk_size
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      INTEGER                                            :: ndo_mo
      LOGICAL                                            :: do_roks, do_uks, quadrants(3)
      REAL(dp), DIMENSION(:, :), POINTER                 :: PQ
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: lhs_int, rhs_int
      TYPE(dbcsr_type)                                   :: work_mat

      NULLIFY (PQ, lhs_int, rhs_int)

      !Initialization
      ndo_mo = donor_state%ndo_mo
      do_roks = xas_tdp_control%do_roks
      do_uks = xas_tdp_control%do_uks
      PQ => xas_tdp_env%ri_inv_ex

      rhs_int => contr1_int
      ALLOCATE (lhs_int(SIZE(contr1_int)))
      CALL copy_ri_contr_int(lhs_int, rhs_int)
      CALL ri_all_blocks_mm(lhs_int, PQ)

      !Given the lhs_int and rhs_int, all we need to do is multiply elements from the former by
      !the transpose of the later, and put the result in the correct spin quadrants

      !Create a normal type work matrix
      CALL dbcsr_create(work_mat, name="WORK", matrix_type=dbcsr_type_no_symmetry, dist=dist, &
                        row_blk_size=blk_size, col_blk_size=blk_size)

      !Case study on closed-shell, ROKS or UKS
      IF (do_roks) THEN
         !In ROKS, the donor MOs for each spin are the same => copy the product in both the
         !alpha-alpha and the beta-beta quadrants
         quadrants = [.TRUE., .FALSE., .TRUE.]
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter, mo_transpose=.TRUE.)

      ELSE IF (do_uks) THEN
         !In UKS, the donor MOs are possibly different for each spin => start with the
         !alpha-alpha product and the perform the beta-beta product separately
         quadrants = [.TRUE., .FALSE., .FALSE.]
         CALL ri_int_product(work_mat, lhs_int(1:ndo_mo), rhs_int(1:ndo_mo), quadrants, &
                             qs_env, eps_filter=xas_tdp_control%eps_filter, mo_transpose=.TRUE.)

         quadrants = [.FALSE., .FALSE., .TRUE.]
         CALL ri_int_product(work_mat, lhs_int(ndo_mo + 1:2*ndo_mo), rhs_int(ndo_mo + 1:2*ndo_mo), &
                             quadrants, qs_env, eps_filter=xas_tdp_control%eps_filter, mo_transpose=.TRUE.)
      ELSE
         !In the restricted closed-shell case, only have one spin and a single qudarant
         quadrants = [.TRUE., .FALSE., .FALSE.]
         CALL ri_int_product(work_mat, lhs_int, rhs_int, quadrants, qs_env, &
                             eps_filter=xas_tdp_control%eps_filter, mo_transpose=.TRUE.)
      END IF
      CALL dbcsr_finalize(work_mat)

      !Create the symmetric kernel matrix and redistribute work_mat into it
      CALL dbcsr_create(offdiag_ex_ker, name="OFFDIAG EX KERNEL", matrix_type=dbcsr_type_symmetric, &
                        dist=dist, row_blk_size=blk_size, col_blk_size=blk_size)
      CALL dbcsr_complete_redistribute(work_mat, offdiag_ex_ker)

      !clean-up
      CALL dbcsr_release(work_mat)
      CALL dbcsr_deallocate_matrix_set(lhs_int)

   END SUBROUTINE offdiag_ex_sc

! **************************************************************************************************
!> \brief Reserves the blocks in of a dbcsr matrix as needed for RI 3-center contraction (aI|P)
!> \param matrices the matrices for which blocks are reserved
!> \param ri_atom the index of the atom on which RI is done (= all coeffs of I are there, and P too)
!> \param qs_env ...
!> \note the end product are normal type matrices that are possibly slightly spraser as matrix_s
! **************************************************************************************************
   SUBROUTINE reserve_contraction_blocks(matrices, ri_atom, qs_env)

      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: matrices
      INTEGER, INTENT(IN)                                :: ri_atom
      TYPE(qs_environment_type), POINTER                 :: qs_env

      INTEGER                                            :: blk, i, iblk, jblk, n
      LOGICAL                                            :: found
      REAL(dp), DIMENSION(:, :), POINTER                 :: pblock_m, pblock_s
      TYPE(dbcsr_distribution_type)                      :: dist
      TYPE(dbcsr_iterator_type)                          :: iter
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: matrix_s
      TYPE(dbcsr_type)                                   :: template, work

      NULLIFY (matrix_s, pblock_s, pblock_m)

!  Initialization
      CALL get_qs_env(qs_env, matrix_s=matrix_s)
      n = SIZE(matrices)
      CALL dbcsr_get_info(matrices(1)%matrix, distribution=dist)

!  Need to redistribute matrix_s in the distribution of matrices
      CALL dbcsr_create(work, template=matrix_s(1)%matrix, dist=dist)
      CALL dbcsr_complete_redistribute(matrix_s(1)%matrix, work)

!  Need to desymmetrize matrix as as a template
      CALL dbcsr_desymmetrize(work, template)

!  Loop over matrix_s as need a,b to overlap
      CALL dbcsr_iterator_start(iter, template)
      DO WHILE (dbcsr_iterator_blocks_left(iter))

         CALL dbcsr_iterator_next_block(iter, row=iblk, column=jblk, blk=blk)
         !only have a,b pair if one of them is the ri_atom
         IF (iblk .NE. ri_atom .AND. jblk .NE. ri_atom) CYCLE

         CALL dbcsr_get_block_p(template, iblk, jblk, pblock_s, found)

         IF (found) THEN
            DO i = 1, n
               NULLIFY (pblock_m)
               CALL dbcsr_reserve_block2d(matrices(i)%matrix, iblk, jblk, pblock_m)
               pblock_m = 0.0_dp
            END DO
         END IF

      END DO !dbcsr iter
      CALL dbcsr_iterator_stop(iter)
      DO i = 1, n
         CALL dbcsr_finalize(matrices(i)%matrix)
      END DO

!  Clean-up
      CALL dbcsr_release(template)
      CALL dbcsr_release(work)

   END SUBROUTINE reserve_contraction_blocks

! **************************************************************************************************
!> \brief Contract the ri 3-center integrals stored in a tensor with repect to the donor MOs coeffs,
!>        for a given excited atom k => (aI|k) = sum_b c_Ib (ab|k)
!> \param contr_int the contracted integrals as array of dbcsr matrices
!> \param op_type for which operator type we contract (COULOMB or EXCHANGE)
!> \param donor_state ...
!> \param xas_tdp_env ...
!> \param xas_tdp_control ...
!> \param qs_env ...
!> \note  In the output matrices, (aI_b|k) is stored at block a,b where I_b is the partial
!>        contraction that only includes coeffs from atom b. Note that the contracted matrix is
!>        not symmetric. To get the fully contracted matrix over b, one need to add the block
!>        columns of (aI_b|k) (get an array of size nao*nsgfp). This step is unnessary in our case
!>        because we assume locality of donor state, and only one column od (aI_b|k) is pouplated
! **************************************************************************************************
   SUBROUTINE contract2_AO_to_doMO(contr_int, op_type, donor_state, xas_tdp_env, xas_tdp_control, qs_env)

      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: contr_int
      CHARACTER(len=*), INTENT(IN)                       :: op_type
      TYPE(donor_state_type), POINTER                    :: donor_state
      TYPE(xas_tdp_env_type), POINTER                    :: xas_tdp_env
      TYPE(xas_tdp_control_type), POINTER                :: xas_tdp_control
      TYPE(qs_environment_type), POINTER                 :: qs_env

      CHARACTER(len=*), PARAMETER :: routineN = 'contract2_AO_to_doMO'

      INTEGER                                            :: handle, i, imo, ispin, katom, kkind, &
                                                            natom, ndo_mo, ndo_so, nkind, nspins
      INTEGER, DIMENSION(:), POINTER                     :: ri_blk_size, std_blk_size
      LOGICAL                                            :: do_uks
      REAL(dp), DIMENSION(:, :), POINTER                 :: coeffs
      TYPE(dbcsr_distribution_type)                      :: opt_dbcsr_dist
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: matrices, matrix_s
      TYPE(dbcsr_type), POINTER                          :: aI_P, P_Ib, work
      TYPE(dbt_type), POINTER                            :: pq_X
      TYPE(distribution_2d_type), POINTER                :: opt_dist2d
      TYPE(gto_basis_set_p_type), DIMENSION(:), POINTER  :: ri_basis
      TYPE(mp_para_env_type), POINTER                    :: para_env
      TYPE(particle_type), DIMENSION(:), POINTER         :: particle_set
      TYPE(qs_kind_type), DIMENSION(:), POINTER          :: qs_kind_set

      NULLIFY (matrix_s, std_blk_size, ri_blk_size, qs_kind_set, ri_basis, pq_X)
      NULLIFY (aI_P, P_Ib, work, matrices, coeffs, opt_dist2d, particle_set)

      CALL timeset(routineN, handle)

!  Initialization
      CALL get_qs_env(qs_env, natom=natom, matrix_s=matrix_s, qs_kind_set=qs_kind_set, para_env=para_env)
      ndo_mo = donor_state%ndo_mo
      kkind = donor_state%kind_index
      katom = donor_state%at_index
      !by default contract for Coulomb
      pq_X => xas_tdp_env%ri_3c_coul
      opt_dist2d => xas_tdp_env%opt_dist2d_coul
      IF (op_type == "EXCHANGE") THEN
         CPASSERT(ASSOCIATED(xas_tdp_env%ri_3c_ex))
         pq_X => xas_tdp_env%ri_3c_ex
         opt_dist2d => xas_tdp_env%opt_dist2d_ex
      END IF
      do_uks = xas_tdp_control%do_uks
      nspins = 1; IF (do_uks) nspins = 2
      ndo_so = nspins*ndo_mo

!  contracted integrals block sizes
      CALL dbcsr_get_info(matrix_s(1)%matrix, col_blk_size=std_blk_size)
      ! getting the block dimensions for the RI basis
      CALL get_qs_env(qs_env, particle_set=particle_set, nkind=nkind)
      ALLOCATE (ri_basis(nkind), ri_blk_size(natom))
      CALL basis_set_list_setup(ri_basis, "RI_XAS", qs_kind_set)
      CALL get_particle_set(particle_set, qs_kind_set, nsgf=ri_blk_size, basis=ri_basis)

!  Create  work matrices. Everything that goes into a 3c routine must be compatible with the optimal dist_2d
      CALL cp_dbcsr_dist2d_to_dist(opt_dist2d, opt_dbcsr_dist)

      ALLOCATE (aI_P, P_Ib, work, matrices(2))
      CALL dbcsr_create(aI_P, dist=opt_dbcsr_dist, matrix_type=dbcsr_type_no_symmetry, name="(aI|P)", &
                        row_blk_size=std_blk_size, col_blk_size=ri_blk_size)

      CALL dbcsr_create(P_Ib, dist=opt_dbcsr_dist, matrix_type=dbcsr_type_no_symmetry, name="(P|Ib)", &
                        row_blk_size=ri_blk_size, col_blk_size=std_blk_size)

      !reserve the blocks (needed for 3c contraction routines)
      matrices(1)%matrix => aI_P; matrices(2)%matrix => P_Ib
      CALL reserve_contraction_blocks(matrices, katom, qs_env)
      DEALLOCATE (matrices)

      ! Create the contracted integral matrices
      ALLOCATE (contr_int(ndo_so))
      DO i = 1, ndo_so
         ALLOCATE (contr_int(i)%matrix)
         CALL dbcsr_create(matrix=contr_int(i)%matrix, template=matrix_s(1)%matrix, &
                           matrix_type=dbcsr_type_no_symmetry, row_blk_size=std_blk_size, &
                           col_blk_size=ri_blk_size)
      END DO

      ! Only take the coeffs for atom on which MOs I,J are localized
      coeffs => donor_state%contract_coeffs

      DO ispin = 1, nspins

!     Loop over the donor MOs and contract
         DO imo = 1, ndo_mo

            ! do the contraction
            CALL dbcsr_set(aI_P, 0.0_dp); CALL dbcsr_set(P_Ib, 0.0_dp)
            CALL contract2_AO_to_doMO_low(pq_X, coeffs(:, (ispin - 1)*ndo_mo + imo), aI_P, P_Ib, katom)

            ! Get the full (aI|P) contracted integrals
            CALL dbcsr_transposed(work, P_Ib)
            CALL dbcsr_add(work, aI_P, 1.0_dp, 1.0_dp)
            CALL dbcsr_complete_redistribute(work, contr_int((ispin - 1)*ndo_mo + imo)%matrix)
            CALL dbcsr_filter(contr_int((ispin - 1)*ndo_mo + imo)%matrix, 1.0E-16_dp)

            CALL dbcsr_release(work)
         END DO !imo
      END DO !ispin

!  Clean-up
      CALL dbcsr_release(aI_P)
      CALL dbcsr_release(P_Ib)
      CALL dbcsr_distribution_release(opt_dbcsr_dist)
      DEALLOCATE (ri_blk_size, aI_P, P_Ib, work, ri_basis)

      CALL timestop(handle)

   END SUBROUTINE contract2_AO_to_doMO

! **************************************************************************************************
!> \brief Contraction of the 3-center integrals (ab|Q) over the RI basis elements Q to get donor MOS
!>        => (ab|IJ) = sum_X (ab|Q) coeffs_Q
!> \param ab_Q the tensor holding the integrals
!> \param vec the contraction coefficients
!> \param mat_abIJ the matrix holding the (ab|IJ) integrals (blocks must be reserved)
!> \param atom_k the atom for which we contract, i.e. we only take RI basis Q centered on atom_k
!> \note By construction, distribution of tensor and matrix match, also for OMP threads
! **************************************************************************************************
   SUBROUTINE contract3_RI_to_doMOs(ab_Q, vec, mat_abIJ, atom_k)

      TYPE(dbt_type)                                     :: ab_Q
      REAL(dp), DIMENSION(:), INTENT(IN)                 :: vec
      TYPE(dbcsr_type)                                   :: mat_abIJ
      INTEGER, INTENT(IN)                                :: atom_k

      CHARACTER(len=*), PARAMETER :: routineN = 'contract3_RI_to_doMOs'

      INTEGER                                            :: handle, i, iatom, ind(3), j, jatom, katom
      LOGICAL                                            :: found, t_found
      REAL(dp)                                           :: prefac
      REAL(dp), ALLOCATABLE, DIMENSION(:, :, :)          :: iabc
      REAL(KIND=dp), DIMENSION(:, :), POINTER            :: pblock
      TYPE(dbcsr_type)                                   :: work
      TYPE(dbt_iterator_type)                            :: iter

      NULLIFY (pblock)

      CALL timeset(routineN, handle)

!$OMP PARALLEL DEFAULT(NONE) &
!$OMP SHARED(ab_Q,vec,mat_abIJ,atom_k) &
!$OMP PRIVATE(iter,ind,iatom,jatom,katom,prefac,iabc,t_found,found,pblock,i,j)
      CALL dbt_iterator_start(iter, ab_Q)
      DO WHILE (dbt_iterator_blocks_left(iter))
         CALL dbt_iterator_next_block(iter, ind)

         iatom = ind(1)
         jatom = ind(2)
         katom = ind(3)

         IF (.NOT. atom_k == katom) CYCLE

         prefac = 1.0_dp
         IF (iatom == jatom) prefac = 0.5_dp

         CALL dbt_get_block(ab_Q, ind, iabc, t_found)

         CALL dbcsr_get_block_p(mat_abIJ, iatom, jatom, pblock, found)
         IF ((.NOT. found) .OR. (.NOT. t_found)) CYCLE

         DO i = 1, SIZE(pblock, 1)
            DO j = 1, SIZE(pblock, 2)
!$OMP ATOMIC
               pblock(i, j) = pblock(i, j) + prefac*DOT_PRODUCT(vec(:), iabc(i, j, :))
            END DO
         END DO

         DEALLOCATE (iabc)
      END DO !iter
      CALL dbt_iterator_stop(iter)
!$OMP END PARALLEL

      !matrix only half filled => need to add its transpose
      CALL dbcsr_create(work, template=mat_abIJ)
      CALL dbcsr_transposed(work, mat_abIJ)
      CALL dbcsr_add(mat_abIJ, work, 1.0_dp, 1.0_dp)
      CALL dbcsr_release(work)

      CALL timestop(handle)

   END SUBROUTINE contract3_RI_to_doMOs

! **************************************************************************************************
!> \brief Contraction of the 3-center integrals over index 1 and 2, for a given atom_k. The results
!>        are stored in two matrices, such that (a,b are block indices):
!>        mat_aIb(ab) = mat_aIb(ab) + sum j_b (i_aj_b|k)*v(j_b) and
!>        mat_bIa(ba) = mat_bIa(ba) + sum i_a (i_aj_b|k)*v(i_a)
!>        The block size of the columns of mat_aIb and the rows of mat_bIa are the size of k (RI)
!> \param ab_Q the tensor containing the 3-center integrals
!> \param vec the contraction coefficients
!> \param mat_aIb normal type dbcsr matrix
!> \param mat_bIa normal type dbcsr matrix
!> \param atom_k the atom for which we contract
!>       It is assumed that the contraction coefficients for MO I are all on atom_k
!>       We do the classic thing when we fill half the matrix and add its transposed to get the full
!>       one, but here, the matrix is not symmetric, hence we explicitely have 2 input matrices
!>       The distribution of the integrals and the normal dbcsr matrix are compatible out of the box
! **************************************************************************************************
   SUBROUTINE contract2_AO_to_doMO_low(ab_Q, vec, mat_aIb, mat_bIa, atom_k)

      TYPE(dbt_type)                                     :: ab_Q
      REAL(dp), DIMENSION(:), INTENT(IN)                 :: vec
      TYPE(dbcsr_type), INTENT(INOUT)                    :: mat_aIb, mat_bIa
      INTEGER, INTENT(IN)                                :: atom_k

      CHARACTER(LEN=*), PARAMETER :: routineN = 'contract2_AO_to_doMO_low'

      INTEGER                                            :: handle, i, iatom, ind(3), j, jatom, &
                                                            katom, s1, s2
      INTEGER, DIMENSION(:), POINTER                     :: atom_blk_size
      LOGICAL                                            :: found, t_found
      REAL(dp), ALLOCATABLE, DIMENSION(:, :, :)          :: iabc
      REAL(dp), DIMENSION(:, :), POINTER                 :: pblock
      TYPE(dbt_iterator_type)                            :: iter

      NULLIFY (atom_blk_size, pblock)

      CALL timeset(routineN, handle)

      CALL dbcsr_get_info(mat_aIb, row_blk_size=atom_blk_size)

!$OMP PARALLEL DEFAULT(NONE) &
!$OMP SHARED(ab_Q,vec,mat_aIb,mat_bIa,atom_k,atom_blk_size) &
!$OMP PRIVATE(iter,ind,iatom,jatom,katom,iabc,t_found,found,s1,s2,j,i,pblock)
      CALL dbt_iterator_start(iter, ab_Q)
      DO WHILE (dbt_iterator_blocks_left(iter))
         CALL dbt_iterator_next_block(iter, ind)

         iatom = ind(1)
         jatom = ind(2)
         katom = ind(3)

         IF (atom_k .NE. katom) CYCLE

         CALL dbt_get_block(ab_Q, ind, iabc, t_found)
         IF (.NOT. t_found) CYCLE

         ! Deal with mat_aIb
         IF (jatom == atom_k) THEN
            s1 = atom_blk_size(iatom)
            s2 = SIZE(iabc, 3)

            CALL dbcsr_get_block_p(matrix=mat_aIb, row=iatom, col=jatom, BLOCK=pblock, found=found)

            IF (found) THEN
               DO i = 1, s1
                  DO j = 1, s2
!$OMP ATOMIC
                     pblock(i, j) = pblock(i, j) + DOT_PRODUCT(vec, iabc(i, :, j))
                  END DO
               END DO
            END IF
         END IF ! jatom == atom_k

         ! Deal with mat_bIa, keep block diagonal empty
         IF (iatom == jatom) CYCLE
         IF (iatom == atom_k) THEN
            s1 = SIZE(iabc, 3)
            s2 = atom_blk_size(jatom)

            CALL dbcsr_get_block_p(matrix=mat_bIa, row=iatom, col=jatom, BLOCK=pblock, found=found)

            IF (found) THEN
               DO i = 1, s1
                  DO j = 1, s2
!$OMP ATOMIC
                     pblock(i, j) = pblock(i, j) + DOT_PRODUCT(vec, iabc(:, j, i))
                  END DO
               END DO
            END IF
         END IF !iatom== atom_k

         DEALLOCATE (iabc)
      END DO !iter
      CALL dbt_iterator_stop(iter)
!$OMP END PARALLEL

      CALL timestop(handle)

   END SUBROUTINE contract2_AO_to_doMO_low

! **************************************************************************************************
!> \brief Multiply all the blocks of a contracted RI integral (aI|P) by a matrix of type (P|...|Q)
!> \param contr_int the integral array
!> \param PQ the smaller matrix to multiply all blocks
!> \note  It is assumed that all non-zero blocks have the same number of columns. Can pass partial
!>        arrays, e.g. contr_int(1:3)
! **************************************************************************************************
   SUBROUTINE ri_all_blocks_mm(contr_int, PQ)

      TYPE(dbcsr_p_type), DIMENSION(:)                   :: contr_int
      REAL(dp), DIMENSION(:, :), INTENT(IN)              :: PQ

      INTEGER                                            :: blk, iblk, imo, jblk, ndo_mo, s1, s2
      LOGICAL                                            :: found
      REAL(dp), ALLOCATABLE, DIMENSION(:, :)             :: work
      REAL(dp), DIMENSION(:, :), POINTER                 :: pblock
      TYPE(dbcsr_iterator_type)                          :: iter

      NULLIFY (pblock)

      ndo_mo = SIZE(contr_int)

      DO imo = 1, ndo_mo
         CALL dbcsr_iterator_start(iter, contr_int(imo)%matrix)
         DO WHILE (dbcsr_iterator_blocks_left(iter))

            CALL dbcsr_iterator_next_block(iter, row=iblk, column=jblk, blk=blk)
            CALL dbcsr_get_block_p(contr_int(imo)%matrix, iblk, jblk, pblock, found)

            IF (found) THEN
               s1 = SIZE(pblock, 1)
               s2 = SIZE(pblock, 2)
               ALLOCATE (work(s1, s2))
               CALL dgemm('N', 'N', s1, s2, s2, 1.0_dp, pblock, s1, PQ, s2, 0.0_dp, work, s1)
               CALL dcopy(s1*s2, work, 1, pblock, 1)
               DEALLOCATE (work)
            END IF

         END DO ! dbcsr iterator
         CALL dbcsr_iterator_stop(iter)
      END DO !imo

   END SUBROUTINE ri_all_blocks_mm

! **************************************************************************************************
!> \brief Copies an (partial) array of contracted RI integrals into anoter one
!> \param new_int where the copy is stored
!> \param ref_int what is copied
!> \note Allocate the matrices of new_int if not done already
! **************************************************************************************************
   SUBROUTINE copy_ri_contr_int(new_int, ref_int)

      TYPE(dbcsr_p_type), DIMENSION(:), INTENT(INOUT)    :: new_int
      TYPE(dbcsr_p_type), DIMENSION(:), INTENT(IN)       :: ref_int

      INTEGER                                            :: iso, ndo_so

      CPASSERT(SIZE(new_int) == SIZE(ref_int))
      ndo_so = SIZE(ref_int)

      DO iso = 1, ndo_so
         IF (.NOT. ASSOCIATED(new_int(iso)%matrix)) ALLOCATE (new_int(iso)%matrix)
         CALL dbcsr_copy(new_int(iso)%matrix, ref_int(iso)%matrix)
      END DO

   END SUBROUTINE copy_ri_contr_int

! **************************************************************************************************
!> \brief Takes the product of contracted integrals and put them in a kernel matrix
!> \param kernel the matrix where the products are stored
!> \param lhs_int the left-hand side contracted integrals
!> \param rhs_int the right-hand side contracted integrals
!> \param quadrants on which quadrant(s) on the kernel matrix the product is stored
!> \param qs_env ...
!> \param eps_filter filter for dbcsr matrix multiplication
!> \param mo_transpose whether the MO blocks should be transpose, i.e. (aI|Jb) => (aJ|Ib)
!> \note It is assumed that the kerenl matrix is NOT symmetric
!>       There are three quadrants, corresponding to 1: the upper-left (diagonal), 2: the
!>       upper-right (off-diagonal) and 3: the lower-right (diagonal).
!>       Need to finalize the kernel matrix after calling this routine (possibly multiple times)
! **************************************************************************************************
   SUBROUTINE ri_int_product(kernel, lhs_int, rhs_int, quadrants, qs_env, eps_filter, mo_transpose)

      TYPE(dbcsr_type), INTENT(INOUT)                    :: kernel
      TYPE(dbcsr_p_type), DIMENSION(:), INTENT(IN)       :: lhs_int, rhs_int
      LOGICAL, DIMENSION(3), INTENT(IN)                  :: quadrants
      TYPE(qs_environment_type), POINTER                 :: qs_env
      REAL(dp), INTENT(IN), OPTIONAL                     :: eps_filter
      LOGICAL, INTENT(IN), OPTIONAL                      :: mo_transpose

      INTEGER                                            :: blk, i, iblk, iso, j, jblk, jso, nblk, &
                                                            ndo_so
      LOGICAL                                            :: found, my_mt
      REAL(dp), DIMENSION(:, :), POINTER                 :: pblock
      TYPE(dbcsr_iterator_type)                          :: iter
      TYPE(dbcsr_p_type), DIMENSION(:), POINTER          :: matrix_s
      TYPE(dbcsr_type)                                   :: prod

      NULLIFY (matrix_s, pblock)

!  Initialization
      CPASSERT(SIZE(lhs_int) == SIZE(rhs_int))
      CPASSERT(ANY(quadrants))
      ndo_so = SIZE(lhs_int)
      CALL get_qs_env(qs_env, matrix_s=matrix_s, natom=nblk)
      CALL dbcsr_create(prod, template=matrix_s(1)%matrix, matrix_type=dbcsr_type_no_symmetry)
      my_mt = .FALSE.
      IF (PRESENT(mo_transpose)) my_mt = mo_transpose

      ! The kernel matrix is symmetric (even if normal type) => only fill upper half on diagonal
      ! quadrants, but the whole thing on upper-right quadrant
      DO iso = 1, ndo_so
         DO jso = 1, ndo_so

            ! If on-diagonal quadrants only, can skip jso < iso
            IF (.NOT. quadrants(2) .AND. jso < iso) CYCLE

            i = iso; j = jso; 
            IF (my_mt) THEN
               i = jso; j = iso; 
            END IF

            ! Take the product lhs*rhs^T
            CALL dbcsr_multiply('N', 'T', 1.0_dp, lhs_int(i)%matrix, rhs_int(j)%matrix, &
                                0.0_dp, prod, filter_eps=eps_filter)

            ! Loop over blocks of prod and fill kernel matrix => ok cuz same (but replicated) dist
            CALL dbcsr_iterator_start(iter, prod)
            DO WHILE (dbcsr_iterator_blocks_left(iter))

               CALL dbcsr_iterator_next_block(iter, row=iblk, column=jblk, blk=blk)
               IF ((iso == jso .AND. jblk < iblk) .AND. .NOT. quadrants(2)) CYCLE

               CALL dbcsr_get_block_p(prod, iblk, jblk, pblock, found)

               IF (found) THEN

                  ! Case study on quadrant
                  !upper-left
                  IF (quadrants(1)) THEN
                     CALL dbcsr_put_block(kernel, (iso - 1)*nblk + iblk, (jso - 1)*nblk + jblk, pblock)
                  END IF

                  !upper-right
                  IF (quadrants(2)) THEN
                     CALL dbcsr_put_block(kernel, (iso - 1)*nblk + iblk, (ndo_so + jso - 1)*nblk + jblk, pblock)
                  END IF

                  !lower-right
                  IF (quadrants(3)) THEN
                     CALL dbcsr_put_block(kernel, (ndo_so + iso - 1)*nblk + iblk, (ndo_so + jso - 1)*nblk + jblk, pblock)
                  END IF

               END IF

            END DO ! dbcsr iterator
            CALL dbcsr_iterator_stop(iter)

         END DO !jso
      END DO !iso

!  Clean-up
      CALL dbcsr_release(prod)

   END SUBROUTINE ri_int_product

END MODULE xas_tdp_kernel
