New semiempirical method of modeling potential energy surfaces for generalized TST and application to the kinetic isotope effects in the CI-H-H system

We present a test of bond-energy-bond-order (BEBO)-type potential energy surfaces for the Cl-H-H reaction system. We consider two methods, the rotated-Morse BEBO (RMBEBO) method and the Agmon-Levine-plus-anti-Morse bend (AL/AB) method. Each method is treated as a one-parameter semiempirical method. The parameter is adjusted to the energy of activation for Cl+H₂ and the test of the method is the comparison of predicted kinetic isotope effects (KIE's) to experiment. Dynamical calculations are carried out by the improved canonical variational transition state theory with Marcus-Coltrin-path semiclassical adiabatic ground-state transmission coefficients as well as with simpler methods. The RMBEBO surfaces are not successful, but the AL/AB method is as successful as previously employed two-parameter methods. The anti-Morse-bend approximation is a general procedure, and it is tested successfully against ab initio calculations.