ACCURATE QUANTUM MONTE-CARLO CALCULATIONS OF THE TUNNELING SPLITTING IN (HF)(2) ON A 6-DIMENSIONAL POTENTIAL HYPERSURFACE

The ground state tunneling splitting in (HF)(2) is calculated with a b asis free diffusion quantum Monte Carlo method in the full vibrational coordinate space as well as in the subspace of hydrogen bond coordina tes (with rigid monomers) for an empirically adjusted ab initio potent ial 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-L (ex periment: 0.66 cm(-1)). Fitting and empirical adjustment effects are s hown to be negligible. Our results stand in marked contrast to recent claims of significant tunneling enhancement by zero-point motion of th e HF monomers, whereas they are in excellent agreement with other avai lable evidence. We also obtain an improved estimate of the dissociatio n energy D-c/hc = 1564 cm(-1) (corresponding to a new D-o/hc = 1057.5 +/- 0.5 cm(-1) on the SQSBDE surface).