Treat each interaction as an isolated two-particle scattering event, neglecting simultaneous interactions involving three or more particles. For a dilute short-range gas, the interaction range is much smaller than the mean free path, making this approximation compatible with a local Boltzmann equation. It is distinct from molecular chaos for the Boltzmann equation, which is the statistical factorization of incoming pair distributions.
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The binary collision approximation (BCA) is a simplified model used in the field of nuclear and particle physics, as well as in materials science, to describe the interactions between particles in a medium. The primary assumption of the BCA is that the collisions between particles occur one at a time and are treated as discrete events, with other particles treated as static or unaffected during these collisions.