Figure shows that the energy barriers are a combination of large positive energy terms -mostly Fermi repulsion between frozen electron densities of the molecules (∆E and geometry distortion of methane (∆E GD (CH 4 )) -and large negative terms arising from intramolecular (∆E P OL ) and intermolecular (∆E CT ) orbital relaxation. The barrier appears because the positive terms are not fully compensated by the negative terms (Figure ).