It follows from this equation that both ⃗ µ and ⃗ m must be non-zero and oriented non-perpendicularly for a transition to be optically active. Often transitions with high R are electrically allowed and therefore driven by large ⃗ µ, which, while enhancing ∆ε, reduces the dissymmetry factor, g = ∆ε/ε, at a given wavelength. Ideally, optically active transitions must be both magnetically and electrically allowed to facilitate both high absolute and relative optical activity as given by ∆ε and g. Electrically allowed transitions have a linear displacement of charge that give rise to ⃗ µ. For a simple electronic transition, e.g., HOMO→LUMO, the transition density is calculated from the direct product of the implicated MOs of the transition, which gives rise to the transition dipole moments. In the π → π * transition of ethene shown in Figure , there is a phase-difference in one end of the molecule. Consequently, charge is depleted in one end and increased in the other end, leading to ⃗ µ pointing along the molecular axis, and the S 0 → S 1 transition of ethene is electric-dipole allowed. Without rotation of charge, ⃗ m is zero. An example of a magnetic-dipole allowed transition is the HOMO→LUMO (n → π * ) transition of formaldehyde shown in Figure . The nodal planes of the HOMO and LUMO have different orientation and the product therefore gives a quadrupole. When the electric quadropole is associated to a rotation of charge it gives rise to the magnetic transition dipole moment. The direction of ⃗ m depends on the direction of the rotation as given by the right-hand rule. The net direction of rotation around the molecular axis is not defined for an achiral species such as formaldehyde, and the direction of ⃗ m in Figure is arbitrarily chosen. This analysis enables us to pinpoint that the S 0 → S 1 transition is optically inactive because it is electric-dipole forbidden. The chiroptical response of a molecule thus depends on the chirality contained in the electronic structure, not specifically the chiral arrangement of atoms in the molecule. There is no direct correspondence between an enantiomer's chiral configuration and the chiroptical sign of its electronic transitions. From the basics of MO theory it is evident that there must be both linear and rotational displacement of charge during a transition if it is to be both electric-and magnetic-dipole allowed. Furthermore, the angle of the dipoles must be as close to (anti-)parallel as possible to optimize the chiroptical response (see equation 1). To achieve this, the transition density must have a helical shape. This is the fundamental theoretical concept that governs electronic circular dichroism and circularly polarized luminescence spectroscopy. The optical properties of molecules are computed using response theory as implemented in time-dependent density functional theory (TD-DFT) in the Gaussian16 code. The rotatory strengths, electric and magnetic transition dipoles associated to each electronic transition are computed directly this way, and we report them in cgs-units to provide scale of the magnitude of the dipole vectors; see SI part 1 for a note on unit conversion. Geometry optimization and single-point computations were carried out using DFT with the ωB97X-D functional and the def2-TZVP basis set as implemented in Gaussian16. Molecules were optimized to the tight criterion using the ultrafine grid setting. To construct torsion profiles, the dihedral angle between end-groups of allene and spiropentadiene structures is changed without further optimization. MOs are plotted with an isosurface value of