At the optimized 90 • structure, 1,2-dichlorospiropentadiene has rectilinear π-MOs. Shown in Figure , the SOMO and doubly-occupied HOMO are split by over 1 eV. Torsion from the perpendicular structure barely affects the HOMO eigenvalue, while the SOMO eigenvalue becomes higher as the MO attains some helicity. Consequently, the total energy is raised by torsion, which is associated to change in the frontier MO energy. In 1,4-dichlorospiropentadiene the SOMO and HOMO are neardegenerate at 90 • . Torsion reduces the eigenvalue of the doublyoccupied HOMO, which leads to the total energy minimum at 113 • . The evolution of the total energy and the frontier MO en- ergy is very similar (Figure ). While structurally quite different, the 1,3-dichloroallene and 1,4-dichlorospiropentadiene cations have very similar frontier MOs that account for their structural changes. We conclude that the torsion in these radical cations is driven by the change of occupation in the helical frontier MOs.