We present diffusion quantum Monte Carlo calculations of the interchange tunneling splitting of ${}^4{\mathrm{He}}_n(\mathrm{HF}{)}_2$ clusters, $n=1\mathrm{}–10$. The tunneling splitting decreases rapidly for $n=1\mathrm{}–4$ clusters, and much more slowly for $n>\mathrm{}4\mathrm{}$. The decrease calculated for ${}^4{\mathrm{He}}_n(\mathrm{HF}{)}_2$ represents $74\%$ of the reduction in the tunneling splitting measured recently for HF dimer in nanodroplets of more than 2000 He atoms. The first four He atoms quench the interchange tunneling very efficiently by virtue of occupying the equatorial ring which encircles the $C_{2h}$ transition state of the tunneling pathway.

• Received 7 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.123401