ML-MCTDH computations were performed with the Heidelberg MCTDH package Ver-sion 8.6.5. For the primitive basis in Eq. ( ) a ten-point harmonic oscillator discrete variable representation was used. The equations of motion were solved employing variable mean fields, together with a 5th-order Runge-Kutta solver. The ML-trees were generated using spectral clustering as implemented in Ref. 85.