GAUGE TRANSFORMATIONS FOR COUPLED-CHANNEL CALCULATIONS APPLIED TO PAIR PRODUCTION IN RELATIVISTIC ATOMIC-COLLISIONS

It is shown that for single-center coupled-channel calculations, it is not possible to remove some unwanted part W' of the interaction by a gauge transformation, even if the coupled equations do not exhibit an explicit dependence on W'. This dependence necessarily exists and ente rs implicitly via the convergence property of the basis expansion. In two-center expansions, an explicit dependence on the gauge function as sociated with W' always remains because of the nonorthogonality of the basis states belonging to different centers in the reaction zone. The observation discussed here and supported by physical examples is rele vant to the assessment of the asymptotic cross-section behavior for el ectron-positron pair production in relativistic atomic collisions.