Abstract
A four-body classical-trajectory Monte Carlo method has been applied to Rydberg-atom-Rydberg-atom scattering problems. Numerical calculations were conducted over the velocity range ve/3 to 3v2 where v2(au)=1/n and n is the principal quantum number of the Rydberg atom. Maximum ionisation occurs at approximately 1.25 ve with the cross sections approximately scaling as pi n4a02. At the lower velocities, a significant fraction of the ion production is due to negative-ion formation.