Exchange approximations in the scattering of slow electrons by polyatomic molecules

The Hara (1967) and asymptotically adjusted versions of the free-electron-gas exchange (FEGE) approximation are compared with the orthogonalisation technique for electron scattering by CH 4 and H 2 O. In general both FEGE approximations are more attractive than the orthogonalisation approximation but there is evidence that the asymptotically adjusted version overestimates the exchange effects at low scattering energies.