Accurate quantum Monte Carlo calculations of the tunneling splitting in (HF)2 on a six-dimensional potential hypersurface

The ground state tunneling splitting in (HF)2 is calculated with a basis free diffusion quantum Monte Carlo method in the full vibrational coordinate space as well as in the subspace of hydrogen bond coordinates (with rigid monomers) for an empirically adjusted ab initio potential energy surface (SQSBDE). The full calculation yields 0.45 ± 0.15 cm−1, whereas the result for rigid monomers is 0.5 ± 0.2 cm−1 (experiment: 0.66 cm−1. Fitting and emperical adjustment effects are shown to be negligible. Our results stand in marked contrast to recent claims of significant tunneling enhancement by zero-point motion of the HF monomers, whereas they are in excellent agreement with other available evidence. We also obtain an improved estimate of the dissociation energy De/hc = 1564 cm−1 (corresponding to a new D0/hc = 1057.5 ± 0.5 cm−1 on the SQSBDE surface).