Abstract
Using the method of binary collisions, the third virial coefficient of He4 at low temperatures is calculated for a potential consisting of an attractive square well and a repulsive core. The shape of this potential is chosen so as to optimize the fit of the second virial coefficient with experimental data. From a comparison of the results with the third virial data the existence of a three-body bound state is inferred. A binding energy of 0.26°K, calculated from the potential, brings the adjusted results and the experimental data into agreement.