Isomer dependence of HF vibrational frequency shift for Ar_nHF (n=4-14) van der Waals clusters: Quantum five-dimensional bound state calculations

The HF vibrational frequency shifts for Ar_nHF van der Waals (vdW) clusters with n=4-14 are predicted to be strongly isomer-specific, providing distinct spectroscopic signatures for different cluster isomers. This represents an extension of our recent studies of the size dependence of the vibrational frequency shift for Ar_nHF clusters [J. Chem. Phys. 101, 6359, 10 181 (1994)]. The HF vibrational frequency shifts calculated for the two or three lowest-energy isomers of each cluster size considered differ by at least a couple of wave numbers. Their relative magnitudes directly reflect the number of Ar atoms that each Ar_nHF isomer has in the first solvation shell around HF. The calculations are performed on pairwise additive intermolecular potential energy surfaces constructed from spectroscopically accurate Ar-Ar and anisotropic Ar-HF potentials. In the frequency shift calculations, the Ar_n subunit is treated as rigid, frozen in the geometry of one of the global or local Ar_nHF minima found previously by simulated annealing [J. Chem. Phys. 100, 7166 (1994)]. The 5D coupled intermolecular vibrational levels of what is now effectively a floppy Ar _n-HF dimer, are calculated highly accurately by the quantum 5D bound state methodology which is described in detail. The 5D vdW vibrational zero-point energy of the Ar_nHF cluster affects significantly the energy gap between various isomers.