Function coulomb_penetrability 

pub fn coulomb_penetrability(l: u32, eta: f64, rho: f64) -> f64

Coulomb penetrability P_L = ρ / (F_L² + G_L²).

Returns 0 if the wave functions cannot be computed (ρ too small, or the Coulomb fractions fail to converge — only occurs far outside the nuclear-physics domain).

SAMMY Reference

coulomb/mrml08.f90 Coulfg output: Pcoul = Xx / (Fc(L)^2 + Gc(L)^2)