In the case of IS → FS reaction paths, such as in H + +e -→ H * , we again see the that the predicted q from finite differences is roughly a third of the value predicted by fitting (due to the hybridised nature of the proton in the IS). An idealised reaction coordinate ω = (0, 0, 1) corresponding to a proton dropping linearly along the surface-normal axis, leads to a closer match between the two methodologies (denoted as alternative ω in Figure ).