Reaction-path analysis of the effect of monomer excitation on the tunneling splitting of the hydrogen fluoride dimer

The tunneling splitting of (HF)₂ has been calculated by semiclassical adiabatic reaction-path methods, employing a small-curvature approximation for the effective mass along the tunneling path. The calculations include all six vibrational degrees of freedom and are based on the partly ab initio, partly semiempirical potential energy surface proposed recently by the authors and Dykstra. The calculations are carried out for both unexcited and excited monomer vibrations, and they correctly predict that excitation of either monomer lowers the tunneling splitting, but they do not explain why the lower-energy monomer stretch is equally as effective as the higher-energy one.