The potential surface for the exchange reaction HF+Hprime;rarr;H+Hprime;F has been investigated with various multiconfiguration selfhyphen;consistent field (SCF) and configuration interaction (CI) wavefunctions. For the first time, nonlinear geometries have been considered. The calculations demonstrate the importance of diffuse functions on the fluorine for describing nonlinear geometries. A qualitative model is presented to explain the need for diffuse functions. The energy barrier for the exchange reaction is calculated to be sim;45 kcal/mole, which is comparable to the values obtained in previousabinitiocalculations on the collinear reaction surface (Refs.1 and 2). More importantly, the calculations show that the saddle point region is very flat, the barrier changing by only 1ndash;2 kcal/mole between collinear (180deg;) and perpendicular (90deg;) geometries. The optimum angle for the transition state geometry is calculated to be 106deg;. Dynamics calculations on the HF+Hprime;rarr;H+Hprime;F reaction have employed semiempirical Londonndash;Eyring ndash;Polanyindash;Sato (LEPS) surfaces that had artifically small energy barriers, as has been pointed out previously. The present calculations show that not only were the barriers for exchange too low, but that the basic shape of the LEPS surfaces was inappropriate. The implications of this result for future dynamics calculations are discussed.
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