Operator formalisms of reactive molecular scattering

The many-body and coupled-channel approaches to molecular collisions are discussed with emphasis on their relative advantages for constructing physical models and for numerical calculations. Some numerical results obtained within the Hilbert--Schmidt expansion are presented for the two-body T operators of H2 in the lowest 1Σ and 3Σ states. An outline is also given of two recently developed approaches to electronic interactions in molecular collisions