pub fn closest_approach(s: Screening, eps: f64, beta: f64) -> f64Expand description
Reduced distance of closest approach x0 for reduced energy eps and
reduced impact parameter beta > 0.
Solves G(x) = 0. G is strictly increasing (both phi/x and 1/x^2
decrease), so the root is unique and bracketed by
[beta, (1/eps + sqrt(1/eps^2 + 4 beta^2))/2]: at x = beta the
potential term makes G < 0, and at the upper end G >= 0 because
phi <= 1. Newton’s method runs inside that bracket; any step that leaves
it is replaced by bisection.