We implemented a systematic procedure for treating the quantal rotations by including all translational and vibrational degrees of freedom for any triatomic bent molecule in any embedded or confined environment, within the MCTDH framework.Fully coupled quantum treatments were employed to investigate unconventional properties in nanoconfined molecular systems. In this way, we facilitate a complete theoretical analysis of the underlying dynamics, that enables to compute the energy levels and the nuclear spin isomers of a single water molecule trapped in a C 60 fullerene cage.The key point lies on the full 9D description of both nuclear and electronic degrees of freedom, as well as a reliable representation of the guest-host interaction. The presence of occluded impurities or inhomogeneities due to noncovalent interactions in the inter-fullerene environment could modify aspects of the potential, causing significant * To whom correspondence should be addressed coupling between otherwise uncoupled modes. Using specific n-mode model potentials, we obtained splitting patterns, that confirm the effects of symmetry breaking observed by experiments in the ground ortho-H 2 O state. Further, our investigation reveals that the first rotationally excited states of the encapsulated ortho-and para-H 2 O have also raised their three-fold degeneracy. In view of the complexity of the problem our results highlight the importance of accurate and computational demanding approaches for building up predictive models for such nanoconfined molecules.

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.

The multiconfiguration time-dependent Hartree (MCTDH) method using a six-dimensional Hamiltonian that includes all rotational and vibrational degrees of freedom and an ab initio potential energy surface was employed to calculate the rovibronic states of the HeBr van der Waals complex. All rotational states of energies within 7 cm with respect to the energy of the linear ground state were calculated without restriction of the total angular momentum. In total, we obtained 500 and 320 rotationally excited states of the ground vibrational T-shaped and linear isomers of the HeBr, respectively, and compared them with those predicted by the rigid rotor model. A thermodynamic model was then introduced to determine the relative stability of the two conformers as a function of the temperature. On the basis of the present results, the linear conformers were found to be energetically more stable than the T-shaped ones by 1.14 cm at T = 0 K, whereas conversion from linear to T-shaped complexes was observed at temperatures above 2.87 K.

We explore the origin of the anomalous splitting of the 1 01 levels reported experimentally for the H 2 O@C 60 endofullerene, in order to give some insight about the physical interpretations of the symmetry breaking observed. We performed fullycoupled quantum computations within the multiconfiguration time-dependent Hartree approach employing a rigorous procedure to handle such computationally challenging problems. We introduce two competing physical models, and discuss the observed unconventional quantum patterns in terms of anisotropy in the interfullerene interactions, caused by the change in the off-center position of the encapsulated water molecules inside the cage or the uniaxial C 60 -cage distortion, arising from noncovalent bonding upon water's encapsulation, or exohedral fullerene perturbations. Our results show that both scenarios could reproduce the experimentally observed rotational degeneracy pattern, although quantitative agreement with the available experimental rotational levels splitting value has been achieved by the model that considers an uniaxial elongation of the C 60 -cage. Such finding supports that the observed symmetry breaking could be mainly caused by the distortion of the fullerene cage. However, as nuclear quantum treatments rely on the underlying interactions, a decisive conclusion hinges on the availability of their improved description, taken into account both endofullerene and exohedral environments, from forthcoming highly demanding electronic structure many-body interaction studies.