The ML-MCTDH approach provides an in-principle numerical exact solution of the timedependent Schrödinger equation. Besides genuinely numerical aspects, e.g. related to the integrator, the accuracy is determined by thresholds provided for the number of SPFs. The efficiency, however, depends on the choice of the ML-tree (see, e.g. Ref. 45). In what follows we adopt the approach of Ref. 46 insofar as we start with a certain correlation matrix whose analysis then provides a means for clustering of strongly correlated modes. In their work, Mendive-Tapia et al., focusing on the vibrational ground state of small protonated water clusters, generated a mode-mode correlation matrix by stochastic sampling of the configuration space. Instead, we will employ information from the full-dimensional nonadiabatic and laser-driven TDH dynamics. Defining a certain target time interval [0 : T ], TDH expectation values of the physical coordinates ⟨Q ξ (t)⟩ are calculated. In a second step, these expectation values are used to determine the Pearson correlation matrix C with elements