Analyzing energy transfer with density-functional theory in real time: Time scales for the energy transfer between B850 bacteriochlorophylls

We present techniques that allow for predicting energy transfer in multichromophoric systems based on density-functional-theory calculations in real-time. Our work respects that the time-dependent density is the basic quantity in density-functional theory. In the approach that we discuss here, the simulations are done for a complete multimolecular system, i.e., do not require an a priori decomposition into subsystems. Yet, our analysis tools allow one to reliably extract energy-transfer times between different regions or constituents of the multimolecular system, the structure of transition densities, and the relative degree of excitation of constituents. We demonstrate our approach by analyzing the excitation-energy transfer between six bacteriochlorophyll molecules from the B850 ring of the light-harvesting complex 2 of the purple bacterium Rhodoblastus acidophilus. Our analysis shows that energy is transferred through this system on a time scale of ∼45 fs. The spectral analysis reveals that mainly two supermolecular excitations drive the energy transfer in this system.