anchor.diff:  
           ! 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, cdet%unocc_list_beta)
  +        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)
  
positive.code:         !        spin-orbitals into which electrons are excited.
          !    f: bit string representation of the determinant we're exciting
          !        from.
          !    unocc_alpha, unocc_beta: integer list of the unoccupied alpha and
          !        beta (respectively) spin-orbitals.
          ! Returns:
          !    pgen: the generation probability of the excitation.  See notes in
          !        spawning.
  
          use basis, only: basis_length, bit_lookup, nbasis
          use system, only: nvirt, nvirt_alpha, nvirt_beta, nalpha, nbeta, nel
          use momentum_symmetry, only: sym_table, inv_sym
  
          real(p) :: pgen
          integer, intent(in) :: ab_sym
          integer(i0), intent(in) :: f(basis_length)
          integer, intent(in) :: unocc_alpha(nvirt_alpha), unocc_beta(nvirt_beta)
  
          integer :: forbidden_excitations, a, b, a_pos, a_el, b_pos, b_el, ka, kb
  
  
negative.code: 
          ! 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)
  
          ! The hubbard model in momentum space is a special case. Connected
          ! non-identical determinants have the following properties:
  