Nonorthogonal distortion function and appearance of gauge field in atomic scattering

In the scattering of electrons or positrons from atoms the target wave func tion is modified by a distortion term which contains the projectile coordin ate as a parameter. It is shown that the close coupling approximation (CCA) with a nonorthogonal distortion term induces a gauge field in a reduced de scription of the projectile dynamics quite analogous to what is obtained in the Born-Oppenheimer approximation for molecules. As an illustration elast ic scattering cross-sections are calculated for positron-hydrogen scatterin g and the results are shown to be in quite good agreement with those obtain ed from more involved studies. (C) 1999 Elsevier Science B.V. All rights re served.