Electron capture for ion-atom collisions at intermediate energies

A simple two-state two-centre atomic expansion method is introduced to obtain electron-capture cross sections for ion-atom collisions at intermediate energies where the projectile velocity is comparable to the characteristic orbital velocity of the electrons. The non-perturbative ab initio theory is capable of predicting inner-shell electron-capture cross sections in accord with experiments. No semi-empirical corrections are required. The region of validity of the method is also discussed.