Note that based on ⟨H S ⟩ it is also possible to get a rough estimate of the extent to which ⟨P iso ⟩ may be modified due to tunneling. Thus, given that the barrier width increases for finite coupling, for q i = -0.9 au a simple one-dimensional parabolic barrier model predicts a reduced tunneling probability. For the other two cases shown in Fig. (a) the gain in average energies outweighs the increased barrier width, resulting in the tunneling probabilities being enhanced. Despite the approximate nature of the above argument for the modulation in the tunneling probabilities, it does perhaps serve towards an understanding of the differences between the classical and quantum ⟨P iso ⟩ seen in Fig. .