Abstract
Equations for approximate wavefunctions describing the scattering of charged particles by atoms are often obtained by using a truncated eigenfunction (or close coupling) expansion. It is shown how allowance can be made for the states omitted in the expansion, by the introduction of suitable second order potentials. The potentials are evaluated in a closure approximation and depend on an effective energy parameter which can be chosen so that the correct long range interaction is obtained in the incident channel. An impact parameter version of the method is introduced and the relationship with the Glauber approximation is discussed.