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:             ! Are (a,b) both unoccupied?
              if (btest(f(b_el), b_pos)) forbidden_excitations = forbidden_excitations + 1
          end do
          do b = 1, nvirt_beta
              kb = unocc_beta(b)/2
              a = 2*sym_table(ab_sym, inv_sym(kb)) - 1
              a_pos = bit_lookup(1,a)
              a_el = bit_lookup(2,a)
              ! Are (a,b) both unoccupied?
              if (btest(f(a_el), a_pos)) forbidden_excitations = forbidden_excitations + 1
          end do
  
          pgen = 2.0_p/(nalpha*nbeta*(nvirt - forbidden_excitations))
  
      end function calc_pgen_hub_k
  
  
negative.code:         !        spin-orbitals into which electrons are excited.
          !    f: bit string representation of the determinant we're exciting
          !        from.
          !    unocc_alpha: integer list of the unoccupied alpha 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_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)
  
          integer :: forbidden_excitations, a, b, a_pos, a_el, b_pos, b_el, ka, kb
  
  