Scanning the region spanned by these new axes results in a map that contains the reactant and product minima of the square scheme. This portion of the PES now encompasses all nuclear motions associated with proton and electron transfer and provides access to the energy profile of the ground state and any electronically excited states of interest. In reference to the conventional square scheme (Figure ), ⃗ p and ⃗ q represent the PT and ET processes, while PCET would be represented by a line connecting the minima of the ground state surface. We next consider the chemical model systems.