Interaction of positronium with helium atoms – the classical treatment of the 5-body collision system Károly Tőkési, Robert D. DuBois and Takeshi Mukoyama Eur. Phys. J. D, 68 9 (2014) 255 Published online: 24 September 2014 DOI: 10.1140/epjd/e2014-40800-9