The Schwinger variational method in electron-atom and electron-molecule scattering theory with polarisation

A method of implementing the Schwinger variational expression in electron-atom and electron-molecule scattering calculations with polarisation included via an ab initio optical potential is described. The Schwinger expression is also used to obtain approximate molecular Hartree-Fock continuum orbitals at arbitrary energies which are needed to calculate the optical potential above the first electron excitation threshold. Explicit closed-form expressions of integrals required for a Gaussian orbital basis are given. The method is applied to low-energy electron-helium scattering with a second-order optical potential.