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 calcula ted by using non-variational transition state theory with Eckart's tun neling correction. The reason is the high computational cost of a larg er amount of information on the potential energy surface of polyatomic systems. The validity of such an approximation is here tested using t he 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 radi cals to calculate the activation barrier, and the one-dimensional Ecka rt potential function fitted to the position and second derivative of the adiabatic curve at the saddle point to calculate Eckart's tunnelin g factor, there was good agreement with the experimental and accurate quantum dynamics values for T > 300 K.