Rotated Morse curve–spline potential function for A+BC reaction dynamics: Application to (Cl, HBr), (F,H2), and (H+,H2)

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Abstract
Methods for the fitting of potential energy functions to discrete potential energy data for triatomic molecules are discussed. Several new criteria for successful fitting functions are proposed, and existing criteria are reviewed. Different semiempirical and empirical fitting functions are analyzed in terms of these criteria. The rotated Morse curve–spline (RMCS) method is developed for general triatomic molecules, and applied to the systems Cl+HBr, F+H2, and H++H2. The detailed surface features are well reproduced for Cl+HBr and F+H2, particularly near the minimum energy path. Overall standard deviations between reference functions and RMCS functions are 0.98 and 0.35 kcal/mole for Cl+HBr and F+H2, respectively. Averaged classical trajectories on the F+H2 surface show good agreement between LEPS and RMCS surfaces over the range 0–10 eV, although there is no point‐for‐point matching of individual trajectories. Relative CPU times for trajectories on the RMCS and LEPS surfaces are 3.3:1. A H+3 potential surface is obtained by the combined use of CNDO/2 energy data and the RMCS fitting function. The resulting surface is scaled to ab initio data by adjustment of the spline functions. It gives a realistic representation of the ab initio surface, except that nonphysical behavior occurs in the outer contours (this was also true for Cl+HBr). The problem of discontinuities arising from the bond angle–bond distance coordinate system is discussed. Possible extensions and limitations of the RMCS method are considered. When Morse curves can describe the asymptotic dissociation channels for A+BC reactions, the RMCS method provides an accurate interpolating function for triatomic potential energy data. Its suitability for classical and quantal studies of reaction dynamics for A+BC systems depends on the magnitude of the discontinuity problem. Computational procedures to study this problem in advance of and during classical trajectories, are given and discussed in detail for F+H2.