The Schwinger and related variational principles in electron-molecule collision processes

An iterative approach to the solution of the Lippmann-Schwinger integral equation for electron-molecule scattering, based on the use of the Schwinger variational principle is outlined. Application of the theory to electron-molecule collisions at the static-exchange level and for use with optical potentials, and a new variational principle for the strong polar molecule case, are discussed. Results of some applications of these methods to a few systems are considered and some results of the use of the method for molecular photoionization are presented. Multichannel extensions of the Schwinger variational principle are discussed and some results as applied to a simple and exactly soluble two-channel model problem presented.