We use diatomicshyphen;inhyphen;molecules theory to estimate the nuclearhyphen;motion contributions to atomndash;diatom potential energy surfaces for reactive systems, with special emphasis on the saddle point region. We examine the reactions of H, O, F, and Cl with H2and H with Cl2. Nuclearhyphen;motion corrections at the saddle point are in the range 0.007ndash;0.07 kcal/mol for cases with one or two hydrogens and classical barrier heights in the range 0.7ndash;3 kcal/mol and are about 0.2 kcal/mol for cases with two hydrogens and classical barrier heights in the range 8ndash;12 kcal/mol. For F+H2with parameters such that the predicted nuclearhyphen;motion correction is 0.03 kcal/mol at the saddle point, the correction is as large as 0.2 kcal/mol elsewhere on the surface. Isotopic substitution of D for H changes the classical barrier height by 0.003ndash;0.1 kcal/mol for the cases studied.
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