Semiempirical three-dimensional potential energy surfaces suitable for both reaction channels of the XH2 system (X = F, Cl)

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Abstract
Three‐dimensional potential energy surfaces for Reactions (1) F+H2→HF+H, (2) H′+HF→H′F+H, (3) H+HCl→H2+Cl and (4) H′+HCl→H′Cl+H were calculated by a modified version of the diatomics‐in‐molecules (DIM) method. In this version a term which incorporates contributions of three‐center molecular integrals neglected by the DIM method is added to the DIM energy. This is the first time that both reaction channels of all of these systems were considered simultaneously. The potential barriers of Reactions (1) and (2) and the difference between the potential barriers (3) and (4) were fitted by adjusting three parameters. The potential barrier of Reaction (3) was then predicted to be 4.9 kcal/mole. The dependence of the barrier heights, saddle points, and other features of the potential energy surfaces on the geometry were investigated. The transition state geometry was proved to be linear for Reactions (1), (3), and (4) and nonlinear for Reaction (2).