MCTDH and ML-MCTDH are usually not employed for tens of picoseconds. Therefore, we should investigate how its wavefunction norm, total energy, and lowest natural populations (quantities that reflect the quality of the integration) perform. The wavefunction norm in both MCTDH and ML-MCTDH calculations remains as unity up to 50 ps, and the total energy deviations are also negligible (less than 0.005 meV). Figures S9-S11 of section 5 in the SI present the lowest natural population of the SPFs as a function of time for different MCTDH and ML-MCTDH simulations. In all four selected MCTDH simulations (with 12, 16, 20, and 24 SPFs), the lowest natural populations of the five combined modes are below the order of 10 -2 , indicating good quality of the calculations. On the other hand, the lowest natural populations for some multilayer SPFs are 10 -2 in ML-MCTDH simulations with 12 and 16 SPFs. For larger ML-MCTDH simulations with 32 and 48 SPFs, the lowest natural populations of the multilayer SPFs are 10 -3 , again suggesting the reasonable quality of the calculations, even after 50 ps.