Efficient evaluation of second-order transition amplitudes by simultaneous propagation over intermediate and final states

A numerical scheme based on multichannel scattering theory is described to treat the second-order processes for which the intermediate and final states should be considered as multichannel scattering wave functions. Numerical computations are done to demonstrate the stability and efficiency of the method. Frequent q-reversals are observed in the second-order transition probabilities of a model system.