When is electronic friction reliable for dynamics at a molecule-metal interface?

We investigate rates of electron transfer for generalized Anderson-Holstein models in the limit of weak molecule-metal coupling, using both surface hopping and electronic friction dynamics in one and two dimensions. Overall, provided there is an external source of friction, electronic friction can sometimes perform well even in the limit of small metal-molecule coupling and capture nonadiabatic effects. However, we show that electronic friction dynamics is likely to fail if there is a competition between nonequivalent pathways. Our conclusions provide further insight into the recent observation by Ouyang et al., [J. Chem. Theory Comput., 2016, 12, 4178] regarding the applicability of Kramer's theory in the adiabatic limit to recover Marcus theory in the nonadiabatic limit.