Torsional Tunneling Splitting in a Water Trimer.

Using a full-dimensional quantum method for nuclei and a new first-principles water potential, we show that the torsional tunneling splitting in a water trimer can be reproduced with accuracy up to ∼1 cm. We quantify the coupling constants of the nuclear quantum states between nonadjacent wells and show that they are the main reason for shifting the quartet-split levels in spectra from a 1:2:1 spacing. This demonstrates the limitation of treatments using simplified models such as the Hückel model and emphasizes the nonlocal nature of the quantum interactions in this system. With such an endeavor, we examine the quality of the water potential developed and provide a rigorous scheme to decipher the experimental spectra with unprecedented accuracy, which is applicable to more general systems.