On the convergence of the two-centre AO close-coupling method in ion - atom collisions

Convergence of the two-centre atomic orbital (TCAO) close-coupling method in ion - atom collisions is carefully examined within the semiclassical impact parameter approximation. Calculations were carried out for protons colliding with H(1s) in the impact energy range 15 - 1000 keV. It is shown that the spurious oscillatory structures in previous large scale TCAO close-coupling calculations are due to the simultaneous use of the short-ranged pseudo-continuum states on both collision centres. By including a large number of pseudo-continuum states only at one centre in the TCAO expansion, we demonstrate that the resulting cross sections for excitation, capture and ionization for p + H(1s) collisions vary smoothly with collision energy and are in better agreement with the experimental data.