On the other hand, convergence concerning 16, 32, and 48 SPFs in ML-MCTDH simulations deteriorates after 5 ps. However, one can notice that increasing the grid size of the primitive basis functions has no pronounced effect on the population profiles, suggesting good convergence in both quantum dynamics methods. The ML-MCTDH dynamics with 𝑛 = 32, 𝑁 = 32 and 𝑛 = 48, 𝑁 = 48, in principle, can be treated as numerically exact ones as they tend to the so-called standard wavefunction expansion, where the wavefunction is expanded directly into the time-dependent primitive basis functions. Since the largest ML-MCTDH simulation with 𝑛 = 48, 𝑁 = 48 is very expensive and has almost attained a plateau at 35 ps, we present an incomplete result. By increasing the number of SPFs in the ML-MCTDH architecture, the population profiles tend to approach MCTDH results.