Topography of potential-energy surfaces. DIM (diatomics-in-molecules) surfaces for F + HF and Cl + HCl

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Abstract
Potential-energy surfaces have been calculated for the F + HF and Cl + HCl reactions using the DIM (diatomics-in-molecules) method with the 10 polyatomic basis functions of 2A′ symmetry arising from the atomic states H(2S) and X(2Pu). The collinear configuration is preferred for both reactions with a potential-energy barrier in the symmetrical configuration of height Eb≈ 45 kJ mol–1. The effect on barrier height of including polyatomic basis functions arising from the ionic states H+(1S) and X–(1S) could not be reliably determined owing to uncertainty in the form of the HX(1∑+) diabatic states at internuclear distances approaching the equilibrium bond length. The electronic structure of the ground-state potential-energy surface and the lower-excited-state potential-energy surfaces is discussed in terms of the contributory valence-bond wavefunctions.