Usually, the Fermi phase shift δF could tell us how hard the potential is -that is, for the soft potential, we would get small δF ≈ 0, whereas for the hard one δF ≈ π/2. Both physical intuition and quantum-chemical data agree on the first positive maximum being placed at the closest carbon atom. Therefore, as we fit both the starting point x0 as well as the Fermi phase shift δF , their values are intertwined with each other, and on its own discrepancies between them do not possess any further meaning. Therefore in this case latter does not reflect any physical characteristic of the material.