Potential energies for the reaction F+H2→HF+H by the random walk method

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Abstract
Ab initio calculations of potential energies have been carried out by the random walk method for a dozen points on the F–H–H potential energy surface in the region of the saddle point of the barrier for the reaction F+H2→HF+H. The fixed‐node method was used with an importance sampling wave function consisting of a scaled RHF function multiplied by Jastrow factors. The total electronic energies obtained are lower by about 40 kcal/mol than those from the most extensive variational calculation and only about 10 kcal/mol above the exact values. Recovery of correlation energy is about 96% complete. A modified LEPS surface fitted to the results yields a classical barrier height of 4.5±0.6 kcal/mol, a value near that predicted by several variational calculations and as much as 3 kcal/mol above the most recent extrapolated and semiempirical estimates. The barrier height might be lower for calculations with improved node locations; but, since little room for error due to node locations remains, the results provide evidence for a high barrier.