Chl-c1 Chl-c2 TD-LC-DFTB (gas-phase) To better understand these findings, one can also have a look at the structural differences between the Chl-a and Chl-c1/c2 molecules as depicted in Fig. with the Chl-c1/c2 molecules lacking the phytyl tail of Chl-a. Furthermore, the presence of an extra unsaturated pyrrole ring within the Mg-porphyrin chlorin ring and an acrylic group connected to the chlorin ring hamper the π-π conjugation in the Chl-c molecules. These structural differences are the main reason behind the blue shift of the Q y excitation energy of the Chl-c1/c2 molecules in gas phase or organic solvents. At the same time, the Chl-c1 and c2 pigments differ in the side chains of the Mg-porphyrin rings with a very small effect on the Q y excitation energy and thus leading to similar energies for these two types of Chl-c molecules. The relative ordering of the Q y excitation energies for Chl-a, c1, and c2 is identical in theory and the experiments as detailed in Table . The DFT/MRCI approach yields the most accurate values for the absolute and relative Q y excitation energies compared to the experimental outcomes. Most interestingly, the order of the excitation energies for Chl-a, c1, and c2 is different between gas phase (or organic solvents) and the FCP complex of P. tricornutum as shown in Fig. where the Chl-c2 molecule has been found to be the most red-shifted pigment within this Chl network.