Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)_n (n = 3, 4) and mixed hydrogen fluoride/water (HF)_n(H₂O)_n (n = 1, 2, 4) clusters

Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)_n with n=3, 4 and mixed clusters of hydrogen fluoride with water (HF)_n(H₂O)_n where n=1, 2. For the (HF)₄(H₂O)₄ complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)_n, good agreement is found with experimental data. This agreement shows that the Møller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)_n(H₂O)_n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.