We have used a Pekeris-type wave function, expressed on a Sturmian basis in terms of perimetric coordinates, in conjunction with the complex coordinate rotation method, to obtain well-converged estimates of the positions and widths of various ${}^{1}S^e$ and ${}^{3}S^e$ resonances of Ps. Calculations employing bases with one and two length scales were performed. We compare our results to earlier estimates. We also discuss the minimum positive charge that a particle of arbitrary mass must possess in order to bind two electrons.

• Received 15 December 2004

DOI:https://doi.org/10.1103/PhysRevA.71.052505