For the degree of concertedness, the alignment of the vector ⃗ p with the line of concertedness is key. We propose to quantify this by measuring the distance between the product minimum and ⃗ p (in a direction perpendicular to ⃗ d) relative to the length of d via the tangent of θ p , see Figure . Fully concerted protonelectron transfer is thus described as a situation where only the motion of the proton is responsible for all the relevant electronic structure changes. Our suggested definition for concertedness uses 1-tan θ p so that maximum concertedness takes the value of 1. Because the ⃗ p and ⃗ q vectors can move independently closer to or farther from the line of concertedness, the degree of coupling and the degree of concertedness are independent measures of the overall proton and electron transfer event and shed light on different aspects of PCET. These definitions of coupling and concertedness result in limiting scenarios for the real-space model of PCET processes. In coupled proton-electron transfer, the PC and ET vectors span a trapezoid region in which coupling between the ET and PT processes can occur, see Figure . The phenol/phenoxy and indol/indolyl cases presented above fall in this regime. Depending on the relative orientation of the PC and ET vectors as well as the curvature of the PES, the metastable intermediates may be accessible. This scenario can thus cover ETPT or PTET mechanisms depending on the relative magnitudes of reorganization energy and the energies of charge-separated species. Orthogonal ⃗ p and ⃗ q are a limiting case, as a larger angle would imply that progress along PC would hinder ET and vice versa.