Convergence and completeness of the pseudostate expansion for proton-hydrogen collisions in two-center close-coupling calculations

The completeness and convergence of the pseudostate representation of continuum states are studied for ionization of atomic hydrogen by proton impact within the framework of the two-center atomic-orbital close-coupling method. Three types of two-center expansions are compared: Pseudocontinuum states are used (a) both on the target and on the projectile, (b) only on the target, and (c) only on the projectile. Satisfactorily consistent ionization cross sections are obtained for the three expansions at intermediate energies, though the latter two show much slower convergence. This implies that pseudocontinuum states on a center can complement the states on another center to some extent even though they are localized. Expansion (c) fails to converge at high energies within a realistic CPU time, while both treatments (b) and (c) reveal a deficiency arising from the lack of symmetry at low energies below a few keV. The converged theoretical ionization cross sections are larger than the available experimental data by 20% at a collision energy of 50 keV.