Ar_nHF (n = 1-4) van der Waals clusters: A quantum Monte Carlo study of ground state energies, structures and HF vibrational frequency shifts

Rigid-body diffusion quantum Monte Carlo (DQMC) calculations of the intermolecular vibrational ground states of Ar_nHF clusters with n = 1-4, for HF v = 0 and v = I, are reported. The intermolecular degrees of freedom of the clusters are treated in full dimensionality, and the best available pairwise additive potential surfaces are used. The calculations yield intermolecular ground state energies, probability distributions of intermolecular coordinates and HF vibrational red-shifts. The vibrationally averaged cluster structures are consistent with experimental geometries. Small but systematic differences between the pairwise additive DQMC red-shifts and experimental values for the n = 2-4 clusters are indicative of the need for the inclusion of nonadditive interactions.