Breaking the 1 cm ^–1 Discrepancy with Experiment Limit in First-Principles Calculations of Water Dimer Vibration–Rotation–Tunneling Spectra

A new water pair potential energy surface (PES), the leading component of intermolecular interactions in condensed water, has been developed. Accuracy of the PES has been significantly improved compared to previous work by using basis sets with the cardinal number increased by one, novel forms of fitting functions, and better sampling of dimer configuration space. The new PES is almost six times more accurate than the best published one in reproducing benchmark energies at stationary points. In the most stringent test of reproducing experimental rovibrational spectra of the water dimer, our PES reduces the discrepancies between theory and experiment from about 1 cm^-1 to 0.36 cm^-1 and leads to reinterpretations of the spectra. This improved PES accuracy should help in elucidating the origins of the anomalous properties of condensed water.