anchor.diff:          use basis, only: basis_fns
           use determinants, only: det_info
           use excitations, only: excit
  -        use fciqmc_data, only: pattempt_single
  +        use fciqmc_data, only: pattempt_single, pattempt_double
           use excitations, only: find_excitation_permutation1, find_excitation_permutation2
           use hamiltonian_molecular, only: slater_condon1_mol_excit, slater_condon2_mol_excit
   
  
positive.code:         ! events.
  
          use basis, only: basis_fns
          use fciqmc_data, only: pattempt_double
          use system, only: nel, nvirt, nvirt_alpha, nvirt_beta, sym0, sym_max
          use point_group_symmetry, only: nbasis_sym_spin, cross_product_pg_sym
  
  
negative.code:             if (allowed_excitation) then
  
                  ! 3b. Probability of generating this excitation.
                  pgen = pattempt_double*calc_pgen_double_mol(ij_sym, connection%to_orb(1), connection%to_orb(2), &
                                              ij_spin, cdet%symunocc)
  
                  ! 4b. Parity of permutation required to line up determinants.
  