The mean distance of closest approach, for two colliding particles, is defined as an average over their relative kinetic energies. It is shown that the mean distance of closest approach thus defined, is formally identical to the effective hard-sphere diameter in the Barker and Henderson perturbation theory of liquids. © 1976.
Del Río, F., & De Lonngi, D. A. (1976). The mean distance of closest approach in the theory of liquids. Physics Letters A, 56(6), 463–464. https://doi.org/10.1016/0375-9601(76)90729-5