We have recently developed a scheme, referred to as Sia analysis, whereby the OR is expressed as a sum of contributions from occupied i to virtual a molecular orbital (MO) transitions. We can determine what physically is driving optical activity and what parts of the molecule are involved by analyzing the movement of charge density described by these transitions. Using this framework, we have investigated the influence of molecular conformation and functionalization on the OR induced by chiral organic molecules. More recently, we used this approach to understand the origin of the large specific rotation in helicenes, and how functionalization affects the optical response. We found that the major contributions come from transitions where density moves along the conjugated π density. However, other transitions, which can be described as the combination of two separate rotations of the orbital density along the two-halves of the a) Electronic mail: tybalduf@ku.edu b) Electronic mail: mcaricato@ku.edu helix, tend to decrease the OR. Functionalization of the helicenes influences the OR in a complex manner, where the strength of the substituent groups and the length of the helix may lead to cooperative or competitive effects that are not easily predicted a priori, but can be interpreted with our decomposition approach. As we showed in Ref. 9, the method is robust, providing a consistent physical interpretation of the OR in both modified velocity gauge (MVG) and length gauge (LG) calculations, regardless of the choice of perturbation used in solving the linear response equations. The definition of Sia is not limited to canonical orbitals, but localized orbitals can be employed as well. Since the localization procedure is essentially free in terms of computational cost, one can perform the Sia analysis with multiple MO bases simultaneously and utilize the basis with the most compact representation of the OR tensor a posteriori.