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

! **************************************************************************************************
!> \par History
!>      Torsions added (DG) 05-Dec-2000
!>      Variable names changed (DG) 05-Dec-2000
!> \author CJM
! **************************************************************************************************
MODULE fist_intra_force

   USE atprop_types,                    ONLY: atprop_type
   USE cell_types,                      ONLY: cell_type,&
                                              pbc
   USE cp_log_handling,                 ONLY: cp_get_default_logger,&
                                              cp_logger_type
   USE distribution_1d_types,           ONLY: distribution_1d_type
   USE kinds,                           ONLY: dp
   USE mol_force,                       ONLY: force_bends,&
                                              force_bonds,&
                                              force_imp_torsions,&
                                              force_opbends,&
                                              force_torsions,&
                                              get_pv_bend,&
                                              get_pv_bond,&
                                              get_pv_torsion
   USE molecule_kind_types,             ONLY: &
        bend_type, bond_type, get_molecule_kind, impr_type, molecule_kind_type, opbend_type, &
        shell_type, torsion_type, ub_type
   USE molecule_types,                  ONLY: get_molecule,&
                                              molecule_type
   USE particle_types,                  ONLY: particle_type
#include "./base/base_uses.f90"

   IMPLICIT NONE

   PRIVATE
   CHARACTER(len=*), PARAMETER, PRIVATE :: moduleN = 'fist_intra_force'
   PUBLIC :: force_intra_control

CONTAINS

! **************************************************************************************************
!> \brief Computes the the intramolecular energies, forces, and pressure tensors
!> \param molecule_set ...
!> \param molecule_kind_set ...
!> \param local_molecules ...
!> \param particle_set ...
!> \param shell_particle_set ...
!> \param core_particle_set ...
!> \param pot_bond ...
!> \param pot_bend ...
!> \param pot_urey_bradley ...
!> \param pot_torsion ...
!> \param pot_imp_torsion ...
!> \param pot_opbend ...
!> \param pot_shell ...
!> \param pv_bond ...
!> \param pv_bend ...
!> \param pv_urey_bradley ...
!> \param pv_torsion ...
!> \param pv_imp_torsion ...
!> \param pv_opbend ...
!> \param f_bond ...
!> \param f_bend ...
!> \param f_torsion ...
!> \param f_ub ...
!> \param f_imptor ...
!> \param f_opbend ...
!> \param cell ...
!> \param use_virial ...
!> \param atprop_env ...
!> \par History
!>      none
!> \author CJM
! **************************************************************************************************
   SUBROUTINE force_intra_control(molecule_set, molecule_kind_set, &
                                  local_molecules, particle_set, shell_particle_set, core_particle_set, &
                                  pot_bond, pot_bend, pot_urey_bradley, pot_torsion, pot_imp_torsion, &
                                  pot_opbend, pot_shell, pv_bond, pv_bend, pv_urey_bradley, pv_torsion, &
                                  pv_imp_torsion, pv_opbend, f_bond, f_bend, f_torsion, f_ub, &
                                  f_imptor, f_opbend, cell, use_virial, atprop_env)

      TYPE(molecule_type), POINTER                       :: molecule_set(:)
      TYPE(molecule_kind_type), POINTER                  :: molecule_kind_set(:)
      TYPE(distribution_1d_type), POINTER                :: local_molecules
      TYPE(particle_type), POINTER                       :: particle_set(:), shell_particle_set(:), &
                                                            core_particle_set(:)
      REAL(KIND=dp), INTENT(INOUT)                       :: pot_bond, pot_bend, pot_urey_bradley, &
                                                            pot_torsion, pot_imp_torsion, &
                                                            pot_opbend, pot_shell
      REAL(KIND=dp), DIMENSION(:, :), INTENT(INOUT)      :: pv_bond, pv_bend, pv_urey_bradley, &
                                                            pv_torsion, pv_imp_torsion, pv_opbend
      REAL(KIND=dp), DIMENSION(:, :), INTENT(INOUT), &
         OPTIONAL                                        :: f_bond, f_bend, f_torsion, f_ub, &
                                                            f_imptor, f_opbend
      TYPE(cell_type), POINTER                           :: cell
      LOGICAL, INTENT(IN)                                :: use_virial
      TYPE(atprop_type), POINTER                         :: atprop_env

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

      INTEGER :: first_atom, handle, i, ibend, ibond, ikind, imol, imul, index_a, index_b, &
         index_c, index_d, iopbend, ishell, itorsion, nbends, nbonds, nimptors, nkind, &
         nmol_per_kind, nopbends, nshell, ntorsions, nub
      LOGICAL                                            :: atener
      REAL(KIND=dp)                                      :: d12, d32, dist, dist1, dist2, energy, &
                                                            fscalar, id12, id32, is32, ism, isn, &
                                                            k2_spring, k4_spring, r2, s32, sm, sn, &
                                                            theta
      REAL(KIND=dp), DIMENSION(3)                        :: b12, b32, g1, g2, g3, gt1, gt2, gt3, &
                                                            gt4, k1, k2, k3, k4, rij, t12, t32, &
                                                            t34, t41, t42, t43, tm, tn
      REAL(KIND=dp), DIMENSION(:), POINTER               :: ener_a
      TYPE(bend_type), POINTER                           :: bend_list(:)
      TYPE(bond_type), POINTER                           :: bond_list(:)
      TYPE(cp_logger_type), POINTER                      :: logger
      TYPE(impr_type), POINTER                           :: impr_list(:)
      TYPE(molecule_kind_type), POINTER                  :: molecule_kind
      TYPE(molecule_type), POINTER                       :: molecule
      TYPE(opbend_type), POINTER                         :: opbend_list(:)
      TYPE(shell_type), POINTER                          :: shell_list(:)
      TYPE(torsion_type), POINTER                        :: torsion_list(:)
      TYPE(ub_type), POINTER                             :: ub_list(:)

      CALL timeset(routineN, handle)
      NULLIFY (logger)
      logger => cp_get_default_logger()

      IF (PRESENT(f_bond)) f_bond = 0.0_dp
      IF (PRESENT(f_bend)) f_bend = 0.0_dp
      IF (PRESENT(f_torsion)) f_torsion = 0.0_dp
      IF (PRESENT(f_imptor)) f_imptor = 0.0_dp
      IF (PRESENT(f_ub)) f_ub = 0.0_dp

      pot_bond = 0.0_dp
      pot_bend = 0.0_dp
      pot_urey_bradley = 0.0_dp
      pot_torsion = 0.0_dp
      pot_imp_torsion = 0.0_dp
      pot_opbend = 0.0_dp
      pot_shell = 0.0_dp

      atener = atprop_env%energy
      IF (atener) ener_a => atprop_env%atener

      nkind = SIZE(molecule_kind_set)
      MOL: DO ikind = 1, nkind
         nmol_per_kind = local_molecules%n_el(ikind)

         DO imol = 1, nmol_per_kind
            i = local_molecules%list(ikind)%array(imol)
            molecule => molecule_set(i)
            molecule_kind => molecule%molecule_kind
            CALL get_molecule_kind(molecule_kind, nbend=nbends, nbond=nbonds, &
                                   nimpr=nimptors, nub=nub, ntorsion=ntorsions, &
                                   nopbend=nopbends, nshell=nshell, &
                                   bond_list=bond_list, ub_list=ub_list, &
                                   bend_list=bend_list, torsion_list=torsion_list, &
                                   impr_list=impr_list, opbend_list=opbend_list, &
                                   shell_list=shell_list)

            CALL get_molecule(molecule, first_atom=first_atom)

            BOND: DO ibond = 1, nbonds
               index_a = bond_list(ibond)%a + first_atom - 1
               index_b = bond_list(ibond)%b + first_atom - 1
               rij = particle_set(index_a)%r - particle_set(index_b)%r
               rij = pbc(rij, cell)
               CALL force_bonds(bond_list(ibond)%bond_kind%id_type, rij, &
                                bond_list(ibond)%bond_kind%r0, &
                                bond_list(ibond)%bond_kind%k, &
                                bond_list(ibond)%bond_kind%cs, &
                                energy, fscalar)
               pot_bond = pot_bond + energy
               IF (atener) THEN
                  ener_a(index_a) = ener_a(index_a) + 0.5_dp*energy
                  ener_a(index_b) = ener_a(index_b) + 0.5_dp*energy
               END IF

               particle_set(index_a)%f(1) = particle_set(index_a)%f(1) - rij(1)*fscalar
               particle_set(index_a)%f(2) = particle_set(index_a)%f(2) - rij(2)*fscalar
               particle_set(index_a)%f(3) = particle_set(index_a)%f(3) - rij(3)*fscalar
               particle_set(index_b)%f(1) = particle_set(index_b)%f(1) + rij(1)*fscalar
               particle_set(index_b)%f(2) = particle_set(index_b)%f(2) + rij(2)*fscalar
               particle_set(index_b)%f(3) = particle_set(index_b)%f(3) + rij(3)*fscalar

               ! computing the pressure tensor
               k2 = -rij*fscalar
               IF (use_virial) CALL get_pv_bond(k2, rij, pv_bond)

               ! the contribution from the bonds. ONLY FOR DEBUG
               IF (PRESENT(f_bond)) THEN
                  f_bond(1, index_a) = f_bond(1, index_a) - rij(1)*fscalar
                  f_bond(2, index_a) = f_bond(2, index_a) - rij(2)*fscalar
                  f_bond(3, index_a) = f_bond(3, index_a) - rij(3)*fscalar
                  f_bond(1, index_b) = f_bond(1, index_b) + rij(1)*fscalar
                  f_bond(2, index_b) = f_bond(2, index_b) + rij(2)*fscalar
                  f_bond(3, index_b) = f_bond(3, index_b) + rij(3)*fscalar
               END IF

            END DO BOND

            SHELL: DO ishell = 1, nshell
               index_a = shell_list(ishell)%a + first_atom - 1
               index_b = particle_set(index_a)%shell_index
               rij = core_particle_set(index_b)%r - shell_particle_set(index_b)%r
               rij = pbc(rij, cell)
               k2_spring = shell_list(ishell)%shell_kind%k2_spring
               k4_spring = shell_list(ishell)%shell_kind%k4_spring
               r2 = DOT_PRODUCT(rij, rij)
               energy = 0.5_dp*(k2_spring + k4_spring*r2/12.0_dp)*r2
               fscalar = k2_spring + k4_spring*r2/6.0_dp
               pot_shell = pot_shell + energy
               IF (atener) THEN
                  ener_a(index_a) = ener_a(index_a) + energy
               END IF
               core_particle_set(index_b)%f(1) = core_particle_set(index_b)%f(1) - rij(1)*fscalar
               core_particle_set(index_b)%f(2) = core_particle_set(index_b)%f(2) - rij(2)*fscalar
               core_particle_set(index_b)%f(3) = core_particle_set(index_b)%f(3) - rij(3)*fscalar
               shell_particle_set(index_b)%f(1) = shell_particle_set(index_b)%f(1) + rij(1)*fscalar
               shell_particle_set(index_b)%f(2) = shell_particle_set(index_b)%f(2) + rij(2)*fscalar
               shell_particle_set(index_b)%f(3) = shell_particle_set(index_b)%f(3) + rij(3)*fscalar
               ! Compute the pressure tensor, if requested
               IF (use_virial) THEN
                  k1 = -rij*fscalar
                  CALL get_pv_bond(k1, rij, pv_bond)
               END IF
            END DO SHELL

            UREY_BRADLEY: DO ibend = 1, nub
               index_a = ub_list(ibend)%a + first_atom - 1
               index_b = ub_list(ibend)%c + first_atom - 1
               rij = particle_set(index_a)%r - particle_set(index_b)%r
               rij = pbc(rij, cell)
               CALL force_bonds(ub_list(ibend)%ub_kind%id_type, rij, &
                                ub_list(ibend)%ub_kind%r0, &
                                ub_list(ibend)%ub_kind%k, 0.0_dp, energy, fscalar)
               pot_urey_bradley = pot_urey_bradley + energy
               IF (atener) THEN
                  ener_a(index_a) = ener_a(index_a) + 0.5_dp*energy
                  ener_a(index_b) = ener_a(index_b) + 0.5_dp*energy
               END IF
               particle_set(index_a)%f(1) = particle_set(index_a)%f(1) - rij(1)*fscalar
               particle_set(index_a)%f(2) = particle_set(index_a)%f(2) - rij(2)*fscalar
               particle_set(index_a)%f(3) = particle_set(index_a)%f(3) - rij(3)*fscalar
               particle_set(index_b)%f(1) = particle_set(index_b)%f(1) + rij(1)*fscalar
               particle_set(index_b)%f(2) = particle_set(index_b)%f(2) + rij(2)*fscalar
               particle_set(index_b)%f(3) = particle_set(index_b)%f(3) + rij(3)*fscalar

               ! computing the pressure tensor
               k2 = -rij*fscalar
               IF (use_virial) CALL get_pv_bond(k2, rij, pv_urey_bradley)

               ! the contribution from the ub. ONLY FOR DEBUG
               IF (PRESENT(f_ub)) THEN
                  f_ub(:, index_a) = f_ub(:, index_a) - rij*fscalar
                  f_ub(:, index_b) = f_ub(:, index_b) + rij*fscalar
               END IF

            END DO UREY_BRADLEY

            BEND: DO ibend = 1, nbends
               index_a = bend_list(ibend)%a + first_atom - 1
               index_b = bend_list(ibend)%b + first_atom - 1
               index_c = bend_list(ibend)%c + first_atom - 1
               b12 = particle_set(index_a)%r - particle_set(index_b)%r
               b32 = particle_set(index_c)%r - particle_set(index_b)%r
               b12 = pbc(b12, cell)
               b32 = pbc(b32, cell)
               d12 = SQRT(DOT_PRODUCT(b12, b12))
               id12 = 1.0_dp/d12
               d32 = SQRT(DOT_PRODUCT(b32, b32))
               id32 = 1.0_dp/d32
               dist = DOT_PRODUCT(b12, b32)
               theta = (dist*id12*id32)
               IF (theta < -1.0_dp) theta = -1.0_dp
               IF (theta > +1.0_dp) theta = +1.0_dp
               theta = ACOS(theta)
               CALL force_bends(bend_list(ibend)%bend_kind%id_type, &
                                b12, b32, d12, d32, id12, id32, dist, theta, &
                                bend_list(ibend)%bend_kind%theta0, &
                                bend_list(ibend)%bend_kind%k, &
                                bend_list(ibend)%bend_kind%cb, &
                                bend_list(ibend)%bend_kind%r012, &
                                bend_list(ibend)%bend_kind%r032, &
                                bend_list(ibend)%bend_kind%kbs12, &
                                bend_list(ibend)%bend_kind%kbs32, &
                                bend_list(ibend)%bend_kind%kss, &
                                bend_list(ibend)%bend_kind%legendre, &
                                g1, g2, g3, energy, fscalar)
               pot_bend = pot_bend + energy
               IF (atener) THEN
                  ener_a(index_a) = ener_a(index_a) + energy/3._dp
                  ener_a(index_b) = ener_a(index_b) + energy/3._dp
                  ener_a(index_c) = ener_a(index_c) + energy/3._dp
               END IF
               particle_set(index_a)%f(1) = particle_set(index_a)%f(1) + g1(1)*fscalar
               particle_set(index_a)%f(2) = particle_set(index_a)%f(2) + g1(2)*fscalar
               particle_set(index_a)%f(3) = particle_set(index_a)%f(3) + g1(3)*fscalar
               particle_set(index_b)%f(1) = particle_set(index_b)%f(1) + g2(1)*fscalar
               particle_set(index_b)%f(2) = particle_set(index_b)%f(2) + g2(2)*fscalar
               particle_set(index_b)%f(3) = particle_set(index_b)%f(3) + g2(3)*fscalar
               particle_set(index_c)%f(1) = particle_set(index_c)%f(1) + g3(1)*fscalar
               particle_set(index_c)%f(2) = particle_set(index_c)%f(2) + g3(2)*fscalar
               particle_set(index_c)%f(3) = particle_set(index_c)%f(3) + g3(3)*fscalar

               ! computing the pressure tensor
               k1 = fscalar*g1
               k3 = fscalar*g3
               IF (use_virial) CALL get_pv_bend(k1, k3, b12, b32, pv_bend)

               ! the contribution from the bends. ONLY FOR DEBUG
               IF (PRESENT(f_bend)) THEN
                  f_bend(:, index_a) = f_bend(:, index_a) + fscalar*g1
                  f_bend(:, index_b) = f_bend(:, index_b) + fscalar*g2
                  f_bend(:, index_c) = f_bend(:, index_c) + fscalar*g3
               END IF
            END DO BEND

            TORSION: DO itorsion = 1, ntorsions
               index_a = torsion_list(itorsion)%a + first_atom - 1
               index_b = torsion_list(itorsion)%b + first_atom - 1
               index_c = torsion_list(itorsion)%c + first_atom - 1
               index_d = torsion_list(itorsion)%d + first_atom - 1
               t12 = particle_set(index_a)%r - particle_set(index_b)%r
               t32 = particle_set(index_c)%r - particle_set(index_b)%r
               t34 = particle_set(index_c)%r - particle_set(index_d)%r
               t43 = particle_set(index_d)%r - particle_set(index_c)%r
               t12 = pbc(t12, cell)
               t32 = pbc(t32, cell)
               t34 = pbc(t34, cell)
               t43 = pbc(t43, cell)
               ! t12 x t32
               tm(1) = t12(2)*t32(3) - t32(2)*t12(3)
               tm(2) = -t12(1)*t32(3) + t32(1)*t12(3)
               tm(3) = t12(1)*t32(2) - t32(1)*t12(2)
               ! t32 x t34
               tn(1) = t32(2)*t34(3) - t34(2)*t32(3)
               tn(2) = -t32(1)*t34(3) + t34(1)*t32(3)
               tn(3) = t32(1)*t34(2) - t34(1)*t32(2)
               sm = SQRT(DOT_PRODUCT(tm, tm))
               ism = 1.0_dp/sm
               sn = SQRT(DOT_PRODUCT(tn, tn))
               isn = 1.0_dp/sn
               s32 = SQRT(DOT_PRODUCT(t32, t32))
               is32 = 1.0_dp/s32
               dist1 = DOT_PRODUCT(t12, t32)
               dist2 = DOT_PRODUCT(t34, t32)
               DO imul = 1, torsion_list(itorsion)%torsion_kind%nmul
                  CALL force_torsions(torsion_list(itorsion)%torsion_kind%id_type, &
                                      s32, is32, ism, isn, dist1, dist2, tm, tn, t12, &
                                      torsion_list(itorsion)%torsion_kind%k(imul), &
                                      torsion_list(itorsion)%torsion_kind%phi0(imul), &
                                      torsion_list(itorsion)%torsion_kind%m(imul), &
                                      gt1, gt2, gt3, gt4, energy, fscalar)
                  pot_torsion = pot_torsion + energy
                  IF (atener) THEN
                     ener_a(index_a) = ener_a(index_a) + energy*0.25_dp
                     ener_a(index_b) = ener_a(index_b) + energy*0.25_dp
                     ener_a(index_c) = ener_a(index_c) + energy*0.25_dp
                     ener_a(index_d) = ener_a(index_d) + energy*0.25_dp
                  END IF
                  particle_set(index_a)%f(1) = particle_set(index_a)%f(1) + gt1(1)*fscalar
                  particle_set(index_a)%f(2) = particle_set(index_a)%f(2) + gt1(2)*fscalar
                  particle_set(index_a)%f(3) = particle_set(index_a)%f(3) + gt1(3)*fscalar
                  particle_set(index_b)%f(1) = particle_set(index_b)%f(1) + gt2(1)*fscalar
                  particle_set(index_b)%f(2) = particle_set(index_b)%f(2) + gt2(2)*fscalar
                  particle_set(index_b)%f(3) = particle_set(index_b)%f(3) + gt2(3)*fscalar
                  particle_set(index_c)%f(1) = particle_set(index_c)%f(1) + gt3(1)*fscalar
                  particle_set(index_c)%f(2) = particle_set(index_c)%f(2) + gt3(2)*fscalar
                  particle_set(index_c)%f(3) = particle_set(index_c)%f(3) + gt3(3)*fscalar
                  particle_set(index_d)%f(1) = particle_set(index_d)%f(1) + gt4(1)*fscalar
                  particle_set(index_d)%f(2) = particle_set(index_d)%f(2) + gt4(2)*fscalar
                  particle_set(index_d)%f(3) = particle_set(index_d)%f(3) + gt4(3)*fscalar

                  ! computing the pressure tensor
                  k1 = fscalar*gt1
                  k3 = fscalar*gt3
                  k4 = fscalar*gt4
                  IF (use_virial) CALL get_pv_torsion(k1, k3, k4, t12, t32, t43, pv_torsion)

                  ! the contribution from the torsions. ONLY FOR DEBUG
                  IF (PRESENT(f_torsion)) THEN
                     f_torsion(:, index_a) = f_torsion(:, index_a) + fscalar*gt1
                     f_torsion(:, index_b) = f_torsion(:, index_b) + fscalar*gt2
                     f_torsion(:, index_c) = f_torsion(:, index_c) + fscalar*gt3
                     f_torsion(:, index_d) = f_torsion(:, index_d) + fscalar*gt4
                  END IF
               END DO
            END DO TORSION

            IMP_TORSION: DO itorsion = 1, nimptors
               index_a = impr_list(itorsion)%a + first_atom - 1
               index_b = impr_list(itorsion)%b + first_atom - 1
               index_c = impr_list(itorsion)%c + first_atom - 1
               index_d = impr_list(itorsion)%d + first_atom - 1
               t12 = particle_set(index_a)%r - particle_set(index_b)%r
               t32 = particle_set(index_c)%r - particle_set(index_b)%r
               t34 = particle_set(index_c)%r - particle_set(index_d)%r
               t43 = particle_set(index_d)%r - particle_set(index_c)%r
               t12 = pbc(t12, cell)
               t32 = pbc(t32, cell)
               t34 = pbc(t34, cell)
               t43 = pbc(t43, cell)
               ! t12 x t32
               tm(1) = t12(2)*t32(3) - t32(2)*t12(3)
               tm(2) = -t12(1)*t32(3) + t32(1)*t12(3)
               tm(3) = t12(1)*t32(2) - t32(1)*t12(2)
               ! t32 x t34
               tn(1) = t32(2)*t34(3) - t34(2)*t32(3)
               tn(2) = -t32(1)*t34(3) + t34(1)*t32(3)
               tn(3) = t32(1)*t34(2) - t34(1)*t32(2)
               sm = SQRT(DOT_PRODUCT(tm, tm))
               ism = 1.0_dp/sm
               sn = SQRT(DOT_PRODUCT(tn, tn))
               isn = 1.0_dp/sn
               s32 = SQRT(DOT_PRODUCT(t32, t32))
               is32 = 1.0_dp/s32
               dist1 = DOT_PRODUCT(t12, t32)
               dist2 = DOT_PRODUCT(t34, t32)
               CALL force_imp_torsions(impr_list(itorsion)%impr_kind%id_type, &
                                       s32, is32, ism, isn, dist1, dist2, tm, tn, t12, &
                                       impr_list(itorsion)%impr_kind%k, &
                                       impr_list(itorsion)%impr_kind%phi0, &
                                       gt1, gt2, gt3, gt4, energy, fscalar)
               pot_imp_torsion = pot_imp_torsion + energy
               IF (atener) THEN
                  ener_a(index_a) = ener_a(index_a) + energy*0.25_dp
                  ener_a(index_b) = ener_a(index_b) + energy*0.25_dp
                  ener_a(index_c) = ener_a(index_c) + energy*0.25_dp
                  ener_a(index_d) = ener_a(index_d) + energy*0.25_dp
               END IF
               particle_set(index_a)%f(1) = particle_set(index_a)%f(1) + gt1(1)*fscalar
               particle_set(index_a)%f(2) = particle_set(index_a)%f(2) + gt1(2)*fscalar
               particle_set(index_a)%f(3) = particle_set(index_a)%f(3) + gt1(3)*fscalar
               particle_set(index_b)%f(1) = particle_set(index_b)%f(1) + gt2(1)*fscalar
               particle_set(index_b)%f(2) = particle_set(index_b)%f(2) + gt2(2)*fscalar
               particle_set(index_b)%f(3) = particle_set(index_b)%f(3) + gt2(3)*fscalar
               particle_set(index_c)%f(1) = particle_set(index_c)%f(1) + gt3(1)*fscalar
               particle_set(index_c)%f(2) = particle_set(index_c)%f(2) + gt3(2)*fscalar
               particle_set(index_c)%f(3) = particle_set(index_c)%f(3) + gt3(3)*fscalar
               particle_set(index_d)%f(1) = particle_set(index_d)%f(1) + gt4(1)*fscalar
               particle_set(index_d)%f(2) = particle_set(index_d)%f(2) + gt4(2)*fscalar
               particle_set(index_d)%f(3) = particle_set(index_d)%f(3) + gt4(3)*fscalar

               ! computing the pressure tensor
               k1 = fscalar*gt1
               k3 = fscalar*gt3
               k4 = fscalar*gt4
               IF (use_virial) CALL get_pv_torsion(k1, k3, k4, t12, t32, t43, pv_imp_torsion)

               ! the contribution from the torsions. ONLY FOR DEBUG
               IF (PRESENT(f_imptor)) THEN
                  f_imptor(:, index_a) = f_imptor(:, index_a) + fscalar*gt1
                  f_imptor(:, index_b) = f_imptor(:, index_b) + fscalar*gt2
                  f_imptor(:, index_c) = f_imptor(:, index_c) + fscalar*gt3
                  f_imptor(:, index_d) = f_imptor(:, index_d) + fscalar*gt4
               END IF
            END DO IMP_TORSION

            OPBEND: DO iopbend = 1, nopbends
               index_a = opbend_list(iopbend)%a + first_atom - 1
               index_b = opbend_list(iopbend)%b + first_atom - 1
               index_c = opbend_list(iopbend)%c + first_atom - 1
               index_d = opbend_list(iopbend)%d + first_atom - 1

               t12 = particle_set(index_a)%r - particle_set(index_b)%r
               t32 = particle_set(index_c)%r - particle_set(index_b)%r
               t34 = particle_set(index_c)%r - particle_set(index_d)%r
               t43 = particle_set(index_d)%r - particle_set(index_c)%r
               t41 = particle_set(index_d)%r - particle_set(index_a)%r
               t42 = pbc(t41 + t12, cell)
               t12 = pbc(t12, cell)
               t32 = pbc(t32, cell)
               t41 = pbc(t41, cell)
               t43 = pbc(t43, cell)
               ! tm = t32 x t12
               tm(1) = t32(2)*t12(3) - t12(2)*t32(3)
               tm(2) = -t32(1)*t12(3) + t12(1)*t32(3)
               tm(3) = t32(1)*t12(2) - t12(1)*t32(2)
               sm = SQRT(DOT_PRODUCT(tm, tm))
               s32 = SQRT(DOT_PRODUCT(t32, t32))
               CALL force_opbends(opbend_list(iopbend)%opbend_kind%id_type, &
                                  s32, tm, t41, t42, t43, &
                                  opbend_list(iopbend)%opbend_kind%k, &
                                  opbend_list(iopbend)%opbend_kind%phi0, &
                                  gt1, gt2, gt3, gt4, energy, fscalar)
               pot_opbend = pot_opbend + energy
               IF (atener) THEN
                  ener_a(index_a) = ener_a(index_a) + energy*0.25_dp
                  ener_a(index_b) = ener_a(index_b) + energy*0.25_dp
                  ener_a(index_c) = ener_a(index_c) + energy*0.25_dp
                  ener_a(index_d) = ener_a(index_d) + energy*0.25_dp
               END IF
               particle_set(index_a)%f(1) = particle_set(index_a)%f(1) + gt1(1)*fscalar
               particle_set(index_a)%f(2) = particle_set(index_a)%f(2) + gt1(2)*fscalar
               particle_set(index_a)%f(3) = particle_set(index_a)%f(3) + gt1(3)*fscalar
               particle_set(index_b)%f(1) = particle_set(index_b)%f(1) + gt2(1)*fscalar
               particle_set(index_b)%f(2) = particle_set(index_b)%f(2) + gt2(2)*fscalar
               particle_set(index_b)%f(3) = particle_set(index_b)%f(3) + gt2(3)*fscalar
               particle_set(index_c)%f(1) = particle_set(index_c)%f(1) + gt3(1)*fscalar
               particle_set(index_c)%f(2) = particle_set(index_c)%f(2) + gt3(2)*fscalar
               particle_set(index_c)%f(3) = particle_set(index_c)%f(3) + gt3(3)*fscalar
               particle_set(index_d)%f(1) = particle_set(index_d)%f(1) + gt4(1)*fscalar
               particle_set(index_d)%f(2) = particle_set(index_d)%f(2) + gt4(2)*fscalar
               particle_set(index_d)%f(3) = particle_set(index_d)%f(3) + gt4(3)*fscalar

               ! computing the pressure tensor
               k1 = fscalar*gt1
               k3 = fscalar*gt3
               k4 = fscalar*gt4

               IF (use_virial) CALL get_pv_torsion(k1, k3, k4, t12, t32, t43, pv_opbend)

               ! the contribution from the opbends. ONLY FOR DEBUG
               IF (PRESENT(f_opbend)) THEN
                  f_opbend(:, index_a) = f_opbend(:, index_a) + fscalar*gt1
                  f_opbend(:, index_b) = f_opbend(:, index_b) + fscalar*gt2
                  f_opbend(:, index_c) = f_opbend(:, index_c) + fscalar*gt3
                  f_opbend(:, index_d) = f_opbend(:, index_d) + fscalar*gt4
               END IF
            END DO OPBEND
         END DO
      END DO MOL

      CALL timestop(handle)

   END SUBROUTINE force_intra_control

END MODULE fist_intra_force

