PCET model: The shapes of the PCET potential surfaces can be represented using an illustrative model (with parameters from Table ). To help develop an intuitive understanding of how the relative orientation of ⃗ p and ⃗ q affects the PES, we take inspiration from the ease with which Marcus-Hush theory models ET processes using simple parabolas and coupling between them. Our model defines parabaloids representing the initial, final, and two charge-separated states, and lets them interact via coupling to realize the final surfaces. The model is freely accessible in an open-source format as an interactive notebook at: git.rwth-aachen.de/ak-krewald/pcet_avocados. It is generated in Cartesian space where the x-, y-, and zaxes are termed "concerted transfer coordinate", "charge separation coordinate" and "energy", with units of Å, Å, and eV, respectively. The inputs required for this model are: the distance between the initial and final state minima (Å), the energy of the minima for all uncoupled states (eV), the angle of the ⃗ p and ⃗ q coordinates (degrees), the spring constants of the paraboloids associated with ⃗ p and ⃗ q coordinates for each state (eV/Å 2 ), and coupling elements between each parabaloid (eV).