In a many-particle configuration, if we consider only pairwise optical interactions among particles and neglect three-body and higher order interactions, we can express the C tensor as an integral over C 2 and the radial distribution function g(r) C(η) = C 0 + 3η 4πa 3 dr C 2 (r) -C 0 g(r) (31) where C 0 = 4πa 3 ε f αI is the C tensor for an isolated particle. Equation ( ) is accurate to O(η) and is only valid for very small η. For a particular particle configuration, we can compute g(r), compute the integral in (31) numerically to get C, and then get the extinction cross-section from (2). For isotropic configurations, this can be expressed as