Cumulative reaction probability, introduced in the collision theory by
W.H.Miller [J. Chem. Phys. 62, 1899 (1975)], characterizes a net effi
ciency of a rearrangement process as a function of the total energy of
the collision system. We derive a formula that expresses this quantit
y in terms of the outgoing wave Green function. Our formula is free fr
om the ambiguities of previous formulations; in particular, no absorbi
ng potentials are required for its implementation. The formula has a p
otentially wide range of applications in atomic and molecular collisio
n physics. As an illustration, we consider the rearrangement processes
in the dt mu system for the energies up to the n = 6 threshold.