Complete characterization of the water dimer vibrational ground state and testing the VRT(ASP-W)III, SAPT-5st, and VRT(MCY-5f) surfaces

We report the observation of extensive a- and c-type rotation-tunnelling (RT) spectra of (H2O)2 for Ka = 0–3, and (D2O)2 for Ka = 0–4. These data allow a detailed characterization of the vibrational ground state to energies comparable to those of the low-lying (70–80 cm−1) intermolecular vibrations. We present a comparison of the experimentally determined molecular constants and tunnelling splittings with those calculated on the VRT(ASP-W)III, SAPT-5st, and VRT(MCY-5f) intermolecular potential energy surfaces. The SAPT-5st potential reproduces the vibrational ground state properties of the water dimer very well. The VRT(MCY-5f) and especially the VRT(ASP-W)III potentials show larger disagreements, in particular for the bifurcation tunnelling splitting.