For the electron transfer (ET, ⃗ q) dimension, we use our recently developed approach for quantifying the Marcus-Hush model from ab initio calculations, a multi-component fitting procedure that satisfies the Marcus-Hush model provides a linear combination of normal modes that describes all nuclear motions relevant to the ET process. These definitions mean that PC and ET are well-defined cuts through the (3N -6)-dimensional PES (nuclear movements expressed in vector form as linear displacements from the equilibrium geometry). Figure shows these motions for the studied model systems. The vector expression reveals that ⃗ p and ⃗ q are not generally orthogonal, see Table . This is easily verified by visual inspection of Figure , where the ET dimension can contain a significant movement of the transferring proton. While no requirement for mutual orthogonality of ⃗ p and ⃗ q can be imposed, orthogonal axes are extremely convenient to map out the spatial degrees of freedom involved in the overall PCET process. Analogously to taking linear combinations of atomic orbitals to form bonding (positive combination) and anti-bonding (negative combination) molecular orbitals, the orthogonality is achieved by taking the sum and the difference of ⃗ p and ⃗ q, see Figure .