anchor.diff:  
           use annihilation, only: direct_annihilation
           use basis, only: basis_length, nbasis
  +        use calc, only: truncation_level
           use determinants, only: det_info, alloc_det_info, dealloc_det_info
           use excitations, only: excit, get_excitation_level
           use energy_evaluation, only: update_energy_estimators
  
positive.code:         !        from.
          !    amplitude: amplitude of cluster.
          !    pcluster: Overall probabilites of selecting this cluster, ie
          !        p_sel.p_s.p_clust.
          ! Out:
          !    nspawn: number of particles spawned.  0 indicates the spawning
          !        attempt was unsuccessful.
  
negative.code:         ! with probability
          !    \tau |<D_s|H|D_s> A_s|
          !    ----------------------
          !       n_sel p_s p_clust
          ! where |D_s> is the determinant formed by applying the excitor to the
          ! reference determinant and A_s is the amplitude.  See comments in
          ! select_cluster about the probabilities.
  
          ! In:
          !    cdet: info on the current excitor (cdet) that we will spawn
          !        from.
          !    amplitude: amplitude of cluster.
          !    pcluster: Overall probabilites of selecting this cluster, ie
          !        n_sel.p_s.p_clust.
  
          use determinants, only: det_info
          use fciqmc_data, only: tau, shift, spawned_pop, H00
  