Untangling Excitonic Energy Transfer for the LHC-II complex from Full First-Principles Calculations. Joaquim Jornet-Somoza1, Joseba Alberdi-Rodríguez2, Bruce Milne3, Xavier Andrade4, Miguel A. L. Marques5, Fernando Nogueira3, Micael J. T. Oliveira6, Angel Rubio2 1) Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain 2) Nano-Bio Spectroscopy group and ETSF Scientific Development Centre, Department of Materials Physics, University of the Basque Country, CFM CSICUPV/EHU-MPC and DIPC, Tolosa Hiribidea 72, E-20018 Donostia-San Sebastián, Spain 3) CFisUC, Department of Physics, University of Coimbra, Rua Larga, 3004–516 Coimbra, Portugal. 4) Lawrence Livermore National Laboratory, Livermore, CA 94550, United States 5) Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, D-06099 Halle, Germany 6) Department of Physics, University of Liège, B-4000, Liège, Belgium Corresponding author: Joaquim Jornet ([email protected]) The current increase in energy demands across the world makes fossil-fuel based energy resources unsustainable. Solar energy conversion is one alternative to produce green renewable energy. However, the efficiency of current organic photovoltaic cells is still low (<10%) compared with natural biological systems (~100%). A deeper understanding of the mechanism that governs the energy transfer inside natural lightharvesting systems will be essential to design highly efficient and bio-inspired devices. In this work we present a full first–principles calculations within the framework of real–space time–dependent density functional theory (TD-DFT) for the complete chlorophyll (Chl) network (~7000 atoms) of the light–harvesting complex from green plants, LHC–II. A local analysis method of the time dependent densities ( (t)) developed for this work has made possible quantum–mechanical studies of the optical response of individual chlorophyll molecules subject to the influence of the remainder of the chromophore network. The site-specific alterations in chlorophyll excitation energies support the existence of distinct energy transfer pathways within the LHC-II complex. We propose a new treatment of the (t) that enables the calculation of the transition densities for big systems, and the exciton energy transfer inside chlorophyll network going beyond Förster’s model.