A semiclassical approach to the calculation of thermal rate constants, based on the flux-flux autocorrelation function method, is presented with its applications. The autocorrelation function is generated along classical trajectories using a classical interpretation of the Boltzmannized flux operator. The activation energies for considered reactions are calculated using the G2/MP2 procedure. The forces are generated using a new parametrization of the PM3 NDDO Hamiltonian optimized for accurate

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... radients. Thermal rate constants for hydrogen abstraction from ethane and haloethanes by hydroxyl radical serve as a first test of this approach. Calculated results are in good agreement with cumulative rate constants for all systems considered over a range of temperature including room temperature. The approach is able to distinguish between ␣ and ␤ abstraction with a result for fluoroethane at room temperature that is consistent with the available experiment and trends that are in line with those expected.