Our goal is to gain insight by assessing the π-MOs and the optical activity of the related ππ * transitions. We initially treat the achiral (D 2d ) allene in quasi-D 2 symmetry in Subsection 3.1. This exercise lets us examine the underlying relation between optical activity and helical MOs without substituents. Furthermore, we gain insight into the limit where allene changes from optically inactive to active when mirror symmetry breaks (cf. ESI Figure when rotation symmetry breaks).