  Instanton rate theory is used to study tunneling events in a wide range of
systems including low-temperature chemical reactions. Despite many successful
applications, the method has never been obtained from first principles, relying
instead on the "ImF" premise. In this paper, the same expression for the rate
of barrier penetration at finite temperature is rederived from quantum
scattering theory [W. H. Miller, S. D. Schwartz, and J. W. Tromp, J. Chem.
Phys. 79, 4889 (1983)] using a semiclassical Green's function formalism. This
justifies the instanton approach and provides a route to deriving the rate of
other processes.
