Abstract
Collision strengths for 1s-2s and 1s-2p excitation of hydrogenic ions of nuclear charge Z are calculated in distorted-wave (DW) and close-coupling (CC) approximations. The main source of error in the DW calculations is the neglect of strong coupling between the 2s and 2p states in calculating the wavefunctions. For near-threshold 1s-2p excitation, the DW calculations are correct to better than 10% only for Z>or=10.

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