Electron collisions with polar molecules: exchange and polarisation in elastic scattering by HCl

The local free-electron-gas exchange potential is shown to be not only superior to the orthogonalisation approximation to exchange, but also to yield results in good agreement with those from an essentially exact treatment of exchange. The imposition of orthogonalisation in addition to the local exchange potential yields a further small improvement in the results. The use of these two approximations in combination appears to be a promising technique. A polarisation potential cut-off at about the position of the H atom yields results for the 2 Sigma eigenphase sum that have resonant behaviour more or less where expected on the basis of measured vibrational excitation cross sections. The elastic differential cross section has a strong peak in the backward direction, with important implications for resonant vibrational excitation.