anchor.diff:              ! Overall probability is the product of this times the number of
               ! ways the cluster could have been selected, ie
               !   n_s!/n_excitors^n_s
  +            ! This is because we consider the cluster t_j t_i and t_i t_j to be
  +            ! the same cluster and hence must account for the number of ways the
  +            ! specific unique cluster can be generated.
               cluster%pselect = (cluster%pselect*factorial(cluster%nexcitors))/(real(tot_walkers-1,p)**cluster%nexcitors)
   
               if (cluster_population /= 0) cluster%excitation_level = get_excitation_level(f0, cdet%f)
  
positive.code: ! either +1 or -1.  Alex Thom defines a_i such that this term is always +1 in his
  ! CCMC papers but this definition is not convenient for computation.  See
  ! comments in collapse_cluster and convert_excitor_to_determinant.
  !
  ! Dynamics in FCIQMC involve stochastically sampling the action of the
  ! Hamiltonian.  As the projection onto the CC wavefunction in (3) involves many
  
negative.code: 
                  ! Normalisation factor for cluster%amplitudes...
                  cluster%amplitude = real(cluster_population,p)/(D0_normalisation**(cluster%nexcitors-1))
              else
                  ! Simply set excitation level to a too high (fake) level to avoid
                  ! this cluster being used.
  