Hydrogen-bonded molecular trimers: Vibration-tunneling states and low-frequency spectra from rigorous high-dimensional quantum calculations

Noncovalently bound molecular trimers, held together by hydrogen-bonded and van der Waals interactions, are the smallest clusters in which the three-body interactions can manifest. Accurate description of these nonadditive interactions is of great importance since they play a crucial role in shaping the energetics, structural, and dynamical properties of condensed phases. Molecular trimers offer a unique opportunity for quantifying the three-body interactions through the comparison of the trimer vibration-tunneling states and spectra computed accurately on a high-level many-body potential energy surface with the high-resolution spectroscopic data. However, until just a couple of years ago, the methodology for high-dimensional fully coupled quantum bound-state calculations of molecular trimers was not available. The situation has changed profoundly as a result of the recent methodological developments described in this perspective. For trimers of diatomic molecules, e.g., (HF)3 and (HCl)3, they have made it possible for the first time rigorous full-dimensional (12D) quantum calculations of their coupled intra- and intermolecular vibrational states. Very recently, this methodology was extended to vibration-rotation states of trimers of diatomics. In a parallel development, the same advances have resulted in a theoretical approach capable of the first rigorous 12D quantum calculations of the intermolecular vibrational states and elaborate tunneling splittings of the water trimer, (H2O)3 and (D2O)3, where the monomers are treated as rigid. The computed eigenstates are utilized in the simulations of realistic low-frequency absorption spectra of water trimers, which can be directly compared with the measured far-infrared spectrum of (H2O)3 in helium nanodroplets.