TRANSITION-STATE THEORY AND ECKARTS TUNNELING FACTOR - A GOOD APPROXIMATION FOR THE CALCULATION OF BIMOLECULAR RATE CONSTANTS

Bimolecular rate constants for polyatomic systems are commonly calculated by using non-variational transition state theory with Eckart's tunneling correction. The reason is the high computational cost of a larger amount of information on the potential energy surface of polyatomic systems. The validity of such an approximation is here tested using the widely studied H + H-2, D + H-2 and H + D2 reactions. With a large basis set, electron correlation, and spin projection for the free radicals to calculate the activation barrier, and the one-dimensional Eckart potential function fitted to the position and second derivative of the adiabatic curve at the saddle point to calculate Eckart's tunneling factor, there was good agreement with the experimental and accurate quantum dynamics values for T > 300 K.