Large basis set and two levels of ab initio calculation (ROHF and MCSCF) are used to determine the electronic structure of reactants, products, and saddle point involved in the hydrogen abstraction reaction H2O2+H-->H-2+HO2. The calculated ROHF and MCSCF imaginary frequency omega(not equal) corresponds to the motion of an hydrogen atom between H2O2 and H and has respectively, a magnitude of 6826.5 and 2909.9 cm-1. Calculated (MP2//ROHF and MP2//MCSCF) values of 8.92 and 7.92 Kcal/mol are, respectively, found for the barrier height of the title reaction. The ab initio results are used with the transition state theory (TST) to evaluate the rate constant k(TST)(T) over the range of temperature 200less than or equal toTless than or equal to2000 K. Tunneling corrections to k(TST)(T) are considered through the evaluation of the transmission coefficient by Wigner (W) and zero curvature tunneling (ZCT) methods. Our results show that the calculated rate constants based on the ROHF electronic structure results do not agree with the experimental values. The best agreement with the preferred experimental values measured by Baulch for 300less than or equal toTless than or equal to800 K and with the values measured by Stang and Hampson for 850less than or equal toTless than or equal to2000 K is found for the calculation based on the MCSCF electronic structure results and using the ZCT method to account for tunneling correction to k(TST). (C) 2003 American Institute of Physics.