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

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Abstract
Zero point energies and structures have been calculated for ArnHF 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 C2v isomer of Ar4HF is predicted to be about 2.8 cm−1 to the red of the C3v isomer band.