(a,b) Torsion-rotation diagrams (reduced energy) [E − BJ(J + 1)] in the ground state, the ν₁₈ = 1 and ν₁₈ = 2 states of torsion for both A-species (left plot) and E-species (right plot). (c) An enlargement of the torsion-rotation diagram for J values between 56 and 70 in order to highlight how energy levels become close to each other. (d) Examples of perturbations between different torsional states (avoided crossings are shown around J = 61 and 62, as well as around J = 66).