Bound-state and finite-collision-time effects in the binary-collision approximation

The kinetic theory for time-correlation functions at low density is studied for potentials with bound states and finite collision times. The contribution to the binary-collision operator from bound pairs of atoms with arbitrarily large interaction times is shown to exist and to vanish for times large compared to the characteristic scattering time, justifying the Boltzmann limit for potentials with attractive parts. The effects of such bound states and finite collision times on the short-time behavior of correlation functions are illustrated by a detailed calculation of the velocity-autocorrelation function for a square-well potential. Good agreement with the corresponding results from molecular dynamics is obtained.