The formally exact expression for the cumulative reaction probability, N(E), was first derived by Miller, Schwartz and Tromp (MST) [J. Chem. Phys. 79, 4889 (1983)] from the flux–flux time-dependent expression for the thermal rate constant. We derive the MST formula for N(E) from the time-independent expression for the state-to-state transition probability. Our proof shows that the flux can be calculated at a dividing surface which can be located at any point, including a point where bound states have nonzero probability density.