HF dimer: a new full-dimensional potential energy surface validated by rigorous 6D quantum calculations of HF-stretch excited intra- and intermolecular vibrational states and tunneling splittings

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We present a new two-body, full-dimensional potential energy surface (PES), referred to as flex-CCpol2025, for the hydrogen fluoride dimer. It was fitted using a fully automated computational protocol to interaction energies computed applying the coupled cluster method with single, double, and noniterative triple excitations. Up to quintuple-zeta basis sets with extrapolations to the complete basis set limit were used, a step beyond the best literature calculations. The training set samples a broader range of dimer coordinate space than the literature ranges. The error of the fit for the grid points with negative total energies is exceptionally low, 0.018 cm−1, while the best published PESs have errors of about 20 cm−1 on the same data. As a rigorous test of accuracy, we carried out 6D fully coupled quantum calculations of HF dimer vibrational spectra, including HF stretch fundamentals, overtones, and their tunneling splittings. The computed energy levels exhibit unprecedented average deviation from experiments of only 0.53 cm−1for the manifold involving monomers in their first excited vibrational states. For monomers in their ground state, the deviations are larger, possibly due to some experimental transitions having fairly large uncertainties. For these transitions, theoretical results can be used to replace experimental ones, also an unprecedented result.Contour plot of the flex-CCpol2025 PES as a function of θ1 and θ2, with all other coordinates fixed at their values at global minimum. The plot reveals a double-well structure with a saddle point at θ1 = 0, θ2 = 0.

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