H2O Trimer: Rigorous 12D Quantum Calculations of Intermolecular Vibrational States, Tunneling Splittings, and Low-Frequency Spectrum

Water trimer, as the smallest water cluster in which the three-body interactions can manifest, is arguably the most important hydrogen-bonded trimer. We present the methodology that for the first time allows rigorous twelve-dimensional (12D) quantum calculation of its intermolecular vibration-tunneling eigenstates, with the monomers treated as rigid. These 12D eigenstates are used to simulate the low-frequency absorption spectrum of the trimer, for direct comparison with the measured far-infrared (FIR) spectrum of water trimer in helium nanodroplets. The 12D calculations reveal weak coupling between the large-amplitude torsional and intermolecular stretching vibrations. The calculated torsional tunneling splittings are in excellent agreement with spectroscopic results. There are visible differences between the spectrum simulated using the 12D eigenstates and that based on our earlier 9D calculations where the stretching vibrations are not included. The peaks in the 12D spectrum are generally shifted to slightly lower energies relative to those in the 9D spectrum, as well as the measured FIR spectrum, and are often split by intermolecular stretch-bend Fermi resonances.