Encapsulation of a Water Molecule inside C(60) Fullerene: The Impact of Confinement on Quantum Features

[Image: see text] 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 C(60) fullerene by means of such a systematic approach that includes all nine degrees of freedom of H(2)O@C(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 H(2)O molecule. On the basis of the present rigorous computations, various aspects of the quantized intermolecular dynamics upon confinement of H(2)O@C(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.