Quantum Monte Carlo calculation of argon–HF clusters: Nonadditive forces, isomerization, and HF frequency shifts

Zero point energies and structures have been calculated for Ar_nHF clusters (n=1–4) using the diffusion quantum Monte Carlo method and potential surfaces based on pairwise additive and non pairwise additive models. Energy differences for related potential surfaces are computed with a correlated sampling technique. The two lowest energy isomers of Ar₄HF are found to be energetically equivalent if zero point energy effects are taken into account. Their energetic ordering depends sensitively on threebody contributions to the interaction potential. The calculated size dependent frequency shifts for fundamental and overtone transitions of HF caused by the argon micromatrix are in good agreement with experimental data but are sensitive to nonadditive forces. The line shifts cannot be explained quantitatively without accounting for zero point energy effects. The band origin of the yet unobserved C_2v isomer of Ar₄HF is predicted to be about 2.8 cm⁻¹ to the red of the C_3v isomer band.