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-region equations for the electron-impact excitation of neutral atoms, and which we have implemented within the computer code STGF. We compare the results 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 obtained in general here points the way to the future extension of related codes for the calculation of energy levels and radiative rates in negative ions, photo-detachment cross sections and free-free electron transition data in the presence of a neutral atom.