Communication: VSCF/VCI vibrational spectroscopy of H7O3+ and H9O4+ using high-level, many-body potential energy surface and dipole moment surfaces.

The vibrational spectra of protonated water clusters continue to be of great interest and a significant challenge to theory. We report high-level, coupled-mode anharmonic (VSCF/VCI) calculations of vibrational spectra of two protonated water clusters, H7O3+ and H9O4+, using the Watson Hamiltonian and new full-dimensional, high-level many-body potential energy and dipole moment surfaces. These many-body representations are first validated against direct CCSD(T)-F12b/aug-cc-pVTZ calculations of the double-harmonic spectra of these clusters. Then they are used with a 4-mode representation of the potential and 18 coupled modes to obtain the anharmonic coupled-mode spectra of these clusters. The calculated spectra agree well with recent Ar-tagged action spectra. In the case of H9O4+, a "side-by-side" comparison shows a very good agreement with virtually every experimental feature.