There is no unique representation of the SOMO and HOMO in the vertical cations of allene and spiropentadiene (cf. Figure ), and both JT distortions are available. Shown in Figure , we study the torsion profile of allene starting from the constrained C 2v and fully optimized D 2 minima. At 90 • the SOMO and HOMO are slightly split for allene (red and blue in top panel). With torsion they cross (an avoided crossing for the C 2v structure) and the doubly-occupied HOMO is stabilized by torsion, while the SOMO is destabilized. In both cases the total frontier MO energies are systematically lowered. Like in the substituted allene, the change of eigenvalues correspond to torsion of the helical frontier MOs (ESI figure ). The MOs of C 2v -allene become helical after a few degrees of torsion, but the total stabilization is bigger for the D 2 -allene when there is torsion, which is seen in the total energy. At 50 • the total frontier MO energies are around 0.1 eV lower by having equal bond lengths (Figure , bottom panel). Driven by the helical frontier MOs, equal bond-lengths give lower total energy at 50 • than with bond-length alternation.