  We revisit the well-known aqueous ferrous-ferric electron transfer reaction
in order to address recent suggestions that nuclear tunnelling can lead to
significant deviation from the linear response assumption inherent in the
Marcus picture of electron transfer. A recent study of this reaction by
Richardson and coworkers has found a large difference between their new
path-integral method, GR-QTST, and the saddle point approximation of Wolynes
(Wolynes theory). They suggested that this difference could be attributed to
the existence of multiple tunnelling pathways, leading Wolynes theory to
significantly overestimate the rate. This was used to argue that the linear
response assumptions of Marcus theory may break down for liquid systems when
tunnelling is important. If true, this would imply that the commonly used
method for studying such systems, where the problem is mapped onto a spin-boson
model, is invalid. However, we have recently shown that size inconsistency in
GR-QTST can lead to poor predictions of the rate in systems with many degrees
of freedom. We have also suggested an improved method, the path-integral linear
golden-rule (LGR) approximation, which fixes this problem. Here we demonstrate
that the GR-QTST results for ferrous-ferric electron transfer are indeed
dominated by its size consistency error. Furthermore, by comparing the LGR and
Wolynes theory results, we confirm the established picture of nuclear
tunnelling in this system. Finally, by comparing our path-integral results to
those obtained by mapping onto the spin-boson model, we reassess the importance
of anharmonic effects and the accuracy of this commonly used mapping approach.
