  Exploiting an approach similar to the R-matrix theory, the diffusion Monte
Carlo method is employed to compute phase shifts and threshold cross sections
for the elastic scattering of o-positronium off light atoms. Results are
obtained for Ps-H and Ps-He as representative cases of open and closed shell
targets. The method allows for an exact treatment of both correlation and
exchange interactions, and represents the most promising approach to deal with
these effects in more complicated targets. In particular the Ps-He threshold
cross section, computed in a many body framework for the first time, represents
a standard by which past and future numerical and experimental estimates can be
judged.
