Positronium-atom collisions

New results are presented for Ps(1s) scattering by H(1s), He(1(1)S) and Li(2s). Calculations have been performed in a coupled state framework, usually employing pseudostates, and allowing for excitation of both the Ps and the atom. In the Ps(1s)-H(1s) calculations the H- formation channel has also been included using a highly accurate H- wave function. Resonances resulting from unstable states in which the positron orbits H- have been calculated and analysed. The new Ps(1s)-He(1(1)S) calculations still fail to resolve existing discrepancies between theory and experiment at very low energies. The possible importance of the Ps(-) formation channel in all three collision systems is discussed. (C) 2004 Elsevier B.V. All rights reserved.