A perturbative approach to the coupled outer-region equations for the electron-impact excitation of neutral atoms

We describe a perturbative approach to the solution of the coupled outer-re gion equations for the electron-impact excitation of neutral atoms, and whi ch we have implemented within the computer code STGF. We compare the result s from STGF for neutral Be and H with those that we have obtained with the non-perturbative R-matrix propagator code FARM. The excellent agreement obt ained in general here points the way to the future extension of related cod es for the calculation of energy levels and radiative rates in negative ion s, photo-detachment cross sections and free-free electron transition data i n the presence of a neutral atom.