If the proton coordinate aligns with the line of concertedness, the overall process is fully concerted according to our definition above since 1-tan 0 ○ = 1. This means that there is no need to additionally stabilize the ET dimension to complete the overall proton and electron transfer event. In other words, whichever electronic structure change accompanies the proton movement will simultaneously lead to electron transfer. We therefore refer to this type of scenario as hydrogen atom transfer. Because the ET vector can move independently, it can be parallel to, at a larger angle, or orthogonal to the PC vector. Turning the attention back to the studied systems: both phenyl/phenoxy and indol/indolyl pairs exhibit the same angle between ⃗ p and ⃗ q, see Table , and should be thus similarly coupled. The indol/indolyl pair, however, features a smaller angle between ⃗ d and ⃗ p which implies a more concerted process. In contrast, the toluene/benzyl pair is weakly coupled but almost fully concerted. We note that all systems lie close to a fully concerted process, see Table . At this point it is not clear how chemical space will map onto the different limiting scenarios, e.g. how likely it is to find a coupled but not concerted process. As a generalization of these observations we propose that a larger degree of concertedness pushes systems towards HAT, while a larger degree of coupling would imply a CPET mechanism.