Table : Q y excitation energies (in eV) of the Chl-a and Chl-c1/c2 molecules based on a gas phase-optimized conformation and in the presence of homogeneous electric fields in directions as indicated and described in the text. The values in brackets show the difference with respect to the results without a field. The maximum change in the excitation energy of the Chl-c molecules is found for the field in the +y direction. As one can see from Table , the excitation energies for the Chl-a molecule are affected by homogeneous fields in the x and y direction and slightly in the z direction. For an electric field in the +x and +y directions, there are noticeable shifts by -0.051 eV and 0.025 eV for the Q y excitation energies, respectively. For the opposite field directions, the largest shift is found for the field in the negative y direction with an energy shift of -0.023 eV. Such energy variations within electric fields produced by the protein environment are actually responsible for the energy ladders in LH complexes such as LHCII , CP29 , and CP43 37 . The picture is, however, different for the Chl-c molecules. Although the pigments again mainly react to electric fields along the y axis (as assigned here), there is almost no effect in the negative y direction and a very large effect in the positive y direction reducing the energies by -0.252 eV and -0.228 eV for Chl-c1 and Chl-c2 molecules, respectively. Physiologically, this outcome is very important for the studied FCP complex since the protein surroundings of the Chl-c pigments reduce the excitation energies of these molecules below those of the Chl-a pigments. Thus, the energy flow will take place on average from the Chl-a to the Chl-c chromophores, i.e., different from what is expected based on the excitation energies in the gas phase and organic solvents . These large shifts in the excitation energies are also reflected in the density differences between ground and excited states shown in Fig. for the positive and in Fig. for the negative field direction.