anchor.diff:  
           ! 2. Calculate the generation probability of the excitation.
           ! For two-band systems this depends only upon the orbitals excited from.
  -        pgen = calc_pgen_hub_k(ij_sym, cdet%f, cdet%unocc_list_alpha, cdet%unocc_list_beta)
  +        pgen = calc_pgen_hub_k(ij_sym, cdet%f, cdet%unocc_list_alpha)
   
           ! The hubbard model in momentum space is a special case. Connected
           ! non-identical determinants have the following properties:
  
positive.code: 
  !--- Excitation generation probabilities ---
  
      pure function calc_pgen_hub_k(ab_sym, f, unocc_alpha, unocc_beta) result(pgen)
  
          ! Calculate the generation probability of a given excitation for the
          ! Hubbard model in momentum space.  The Hubbard model is a special case
  
negative.code: 
          ! 3. Calculate the generation probability of the excitation.
          ! For two-band systems this depends only upon the orbitals excited from.
          pgen = calc_pgen_hub_k(ij_sym, cdet%f, cdet%unocc_list_alpha)
  
          ! 4. find the connecting matrix element.
          call slater_condon2_hub_k_excit(cdet%f, connection, hmatel)
  