 Consider the partitioning $${P_{\ensuremath{\mathsf{c}}\xspace}^{\pi_{\ensuremath{\mathsf{c}}\xspace}}}= \begin{pmatrix} Q & B\\ C & D \end{pmatrix}, \qquad H = \begin{pmatrix} h_q \\ h_b \end{pmatrix}$$ where the blocks $Q, D$ describe the interaction among non-bottleneck and bottleneck states within [cluster]{} ${\ensuremath{\mathsf{c}}\xspace}$ respectively. 