  We present a precise theoretical prediction for the decay width of the bound
state of two electrons and a positron (a negative positronium ion), Gamma(Ps^-)
= 2.087 085(12)/ns. We include O(alpha^2) effects of hard virtual photons as
well as soft corrections to the wave function and the decay amplitude. An
outcome of a large-scale variational calculation, this is the first result for
second-order corrections to a decay of a three-particle bound state. It will be
tested experimentally in the new positronium-ion facility in Garching in
Germany.
