anchor.diff:          ! symmetry).  This is less safe that slater_condon1_mol but much faster
           ! as it allows symmetry checking to be skipped in the integral lookups.
   
  -        use molecular_integrals, only: get_one_body_int_mol_nonzero, get_two_body_int_mol_nonzero
  +        use molecular_integrals, only: get_one_body_int_mol_nonzero, get_two_body_int_mol_nonzero, &
  +                                       get_two_body_exchange_pbc_int_nonzero
           use system, only: sys_t
           use hamiltonian_data, only: hmatel_t
   
  
positive.code:                 do jel = iel+1, sys%nel
                      j = occ_list(jel)
                      hmatel = hmatel + get_two_body_int_mol_nonzero(coulomb_ints, i, j, i, j, sys%basis%basis_fns)
                      if (sys%basis%basis_fns(i)%Ms == sys%basis%basis_fns(j)%Ms) &
                                    hmatel = hmatel - get_two_body_int_mol_nonzero(coulomb_ints, i, j, j, i, sys%basis%basis_fns)
                  end do
              end do
          end associate
  
negative.code:         !    <D|H|D>, the diagonal Hamiltonian matrix element involving D for
          !    systems defined by integrals read in from an FCIDUMP file.
  
          use molecular_integrals, only: get_one_body_int_mol_nonzero, get_two_body_int_mol_nonzero, &
                                          get_two_body_exchange_pbc_int_nonzero
          use system, only: sys_t
  
          real(p) :: hmatel
  