  The orthopositronium decay to three photons is studied in the phase-space
region where one of the photons has an energy comparable to the relative
three-momentum of the e+e- system (w ~ m alpha). The NRQED computation in this
regime shows that the dominant contribution arises from distances ~
1/(mw)^(1/2), which allows to treat the Coulomb interaction perturbatively. The
small-photon energy expansion of the 1-loop decay spectrum from full QED yields
the same result as the effective theory. By doing the threshold expansion of
the 1-loop QED amplitude we confirm that the leading term arises from a
loop-momentum region where q^0 ~ q^2/m ~ w. This corresponds to a new
non-relativistic loop-momentum region, which has to be taken into account for
the description of a non-relativistic particle-antiparticle system that decays
through soft photon emission.
