π−p → γγn at threshold

We discuss perturbation theory estimates (in a pseudoscalar-coupled pion-nucleon theory) for near threshold π^?p → γγn, both from atomic L = 0 and L = 1 orbits. The comparison of various contributing amplitudes (including the Δ resonance contributions) is discussed so as to ascertain which ones dominate, at least in c.m. transverse gauge. A detailed calculation (at threshold) gives a predicted total branching ratio of 2γ to 1γ rates as 1.31 × 10^?4, and also exhibits the γ-γ angular distribution. Significant differences from previous literature are elucidated.