We introduce an efficient quantum fully coupled computational scheme within the multiconfiguration time-dependent Hartree (MCTDH) approach to handle the otherwise extremely costly computations of translational–rotational–vibrational states and energies of light-molecule endofullenes. Quantum calculations on energy levels are reported for a water molecule inside C₆₀ fullerene by means of such a systematic approach that includes all nine degrees of freedom of H₂[email protected]₆₀ and does not consider restrictions above them. The potential energy operator is represented as a sum of natural potentials employing the n-mode expansion, along with the exact kinetic energy operator, by introducing a set of Radau internal coordinates for the H₂O molecule. On the basis of the present rigorous computations, various aspects of the quantized intermolecular dynamics upon confinement of H₂[email protected]₆₀ are discussed, such as the rotational energy level splitting and the significant frequency shifts of the encapsulated water molecule vibrations. The impact of water encapsulation on quantum features is explored, and insights into the nature of the underlying forces are provided, highlighting the importance of a reliable first-principles description of the guest–host interactions.

A full-dimensional quantum dynamics study is carried out for the highly fluxional \documentclass[12pt]{minimal}\begin{document}${\rm H}_5^+$\end{document}H5+ cation on a recent reference potential energy surface by using the multi configuration time-dependent Hartree method. With five equivalent light atoms and shallow barriers between various low-lying stationary points on the surface, the spectroscopic characterization of \documentclass[12pt]{minimal}\begin{document}${\rm H}_5^+$\end{document}H5+ represents a huge challenge for accurate quantum dynamics simulations. The present calculation is the first such a study on this cation, which together with its isotope analogies are of primary importance in the interstellar chemistry. The vibrational ground state properties and several vibrationally excited states corresponding to low vibrational frequency motions, not yet directly observable by the experiment, are presented and analyzed.