Anharmonic vibrational spectroscopy of the F^-(H₂O)_n complexes, n = 1, 2

We report anharmonic vibrational spectra (fundamentals, first overtones) for the F^-(H₂O) and F^-(H₂O)₂ clusters computed at the MP2 and CCSD(T) levels of theory with basis sets of triple-ζ quality. Anharmonic corrections were estimated via the correlation-corrected vibration self-consistent field (CC-VSCF) method. The CC-VSCF anharmonic spectra obtained on the potential energy surfaces evaluated at the CCSD(T) level of theory are the first ones reported at a correlated level beyond MP2. We have found that the average basis set effect (TZP vs aug-cc-pVTZ) is on the order of 30-40 cm^-1, whereas the effects of different levels of electron correlation [MP2 vs CCSD(T)] are smaller, 20-30 cm^-1. However, the basis set effect is much larger in the case of the H-bonded O-H stretch of the F^-(H₂O) cluster amounting to 100 cm^-1 for the fundamentals and 200 cm^-1 for the first overtones. Our calculations are in agreement with the limited available set of experimental data for the F^-(H₂O) and F^-(H₂O)₂ systems and provide additional information that can guide further experimental studies.