The optimal separable basis methodology is suggested for the efficient quantum-mechanical calculation of reactive and inelastic scattering amplitudes. The method gives rise to an optimally convergent distorted wave Born expansion of the Green's function. Computationally, this corresponds to an optimized DVR matrix preconditioning scheme. State-to-state and cumulative reactive scattering results are presented for the benchmark collinear H + H2 → H2 + H system. CPU time and memory requirements are drastically reduced in comparison with other methods, and results are obtained for previously prohibitive energy regimes.