  Although Marcus theory is widely used to describe charge transfer in
molecular systems, in its usual form it is restricted to transfer from one
molecule to another. If a charge is delocalised across multiple donor
molecules, this approach requires us to treat the entire donor aggregate as a
unified supermolecule, leading to potentially expensive quantum-chemical
calculations and making it more difficult to understand how the aggregate
components contribute to the overall transfer. Here, we show that it is
possible to describe charge transfer between groups of molecules in terms of
the properties of the constituent molecules and couplings between them,
obviating the need for expensive supermolecular calculations. We use the
resulting theory to show that charge delocalisation between molecules in either
the donor or acceptor aggregates can enhance the rate of charge transfer
through a process we call supertransfer (or suppress it through subtransfer).
The rate can also be enhanced above what is possible with a single molecule by
judiciously tuning energy levels and reorganisation energies. We also describe
bridge-mediated charge transfer between delocalised molecular aggregates. The
equations of generalised Marcus theory are in closed form, providing
qualitative insight into the impact of delocalisation on charge dynamics in
molecular systems.
