No abstract

Organic molecules store the energy of absorbed light in the form of charge-neutral molecular excitationsFrenkel excitons. Usually, in amorphous organic materials, excitons are viewed as quasiparticles, localized on single molecules, which diffuse randomly through the structure. However, the picture of incoherent hopping is not applicable to some classes of molecular aggregates -assemblies of molecules that have strong near-field interaction between electronic excitations in the individual subunits. Molecular aggregates can be found in nature, in photosynthetic complexes of plants and bacteria, and they can also be produced artificially in various forms including quasi-one dimensional chains, two-dimensional films, tubes, etc. In these structures light is absorbed collectively by many mole cules and the following dynamics of molecular excitation possesses coherent properties. This energy transfer mechanism, mediated by the coherent exciton dynamics, resembles the propagation of electromagnetic waves through a structured medium on the nanometer scale. The absorbed energy can be transferred resonantly over distances of hundreds of nanometers before exciton relaxation occurs. Furthermore, the spatial and energetic landscape of molecular aggregates can enable the funneling of the exciton energy to a small number of molecules either within or outside the aggregate. In this review we establish a bridge between the fields of photo nics and excitonics by describing the present understanding of exciton dynamics in molecular aggregates.

We investigate on the procedure of extracting a "spectral density" from mixed QM/MM calculations and employing it in open quantum systems models. In particular, we study the connection between the energy gap correlation function extracted from ground state QM/MM and the bath spectral density used as input in open quantum system approaches. We introduce a simple model which can give intuition on when the ground state QM/MM propagation will give the correct energy gap. We also discuss the role of higher order correlators of the energy-gap fluctuations which can provide useful information on the bath. Further, various semiclassical corrections to the spectral density, are applied and investigated. Finally, we apply our considerations to the photosynthetic Fenna-Matthews-Olson complex. For this system, our results suggest the use of the Harmonic prefactor for the spectral density rather than the Standard one, which was employed in the simulations of the system carried out to date.

We derive a hierarchy of stochastic evolution equations for pure states (quantum trajectories) to efficiently solve open quantum system dynamics with non-Markovian structured environments. From this hierarchy of pure states (HOPS) the exact reduced density operator is obtained as an ensemble average. We demonstrate the power of HOPS by applying it to the Spin-Boson model, the calculation of absorption spectra of molecular aggregates and energy transfer in a photosynthetic pigment-protein complex.The treatment of the dynamics of realistic open quantum systems still poses both conceptual and computational challenges. These arise from non-Markovian behavior due to a structured environment or strong systemenvironment interaction [1,2]. Severe assumptions, like weak-coupling or Markov approximation, are often made for practical reasons. However, they fail for many systems of interest. In these situations one relies on computationally demanding numerical methods. Among these are path integral approaches [3,4] or hierarchical equations of motion [5,6] for the system's reduced density matrix.In this Letter we follow a different strategy and derive a hierarchy of stochastic differential equations for pure states in the system Hilbert space (quantum trajectories). From this hierarchy of pure states (HOPS) the exact reduced density operator is obtained as an ensemble average. Our approach is based upon non-Markovian Quantum State diffusion (NMQSD), derived in its general form in Refs. [7][8][9][10]. NMQSD has been applied to various physical problems including the description of energy transfer in photosynthesis [11,12]. On a more fundamental side, NMQSD has been studied in the context of continuous measurement theory [13,14] and spontaneous wavefunction localization [15]. Other stochastic approaches, with various levels of applicability have been suggested [16][17][18].Although the NMQSD approach is formally exact, it seemed numerically difficult to handle, because of the appearance of a functional derivative with respect to a stochastic process. Only a few exactly solvable models are known (see e.g. [19][20][21][22]). In previous works we have replaced that functional derivative by an operator ansatz and dealt with it in the so called ZOFE approximation [11,[23][24][25] that allows for a very efficient numerical solution and agrees remarkably well with established results for a large number of problems. However, in certain cases this method is known to fail (see e.g. [11,25]). In Ref.[26] a hierarchical approach is applied to the operator ansatz of the functional derivative. Our new HOPS presented here is not based on the previously assumed ansatz, it is numerically exact, converges rapidly and offers a systematic way to check for convergence by increasing the number of equations taken into account. In addition, it offers the advantages of stochastic Schrödinger equations, e.g. one deals with pure states (and not large density matrices) and the calculation of independent realizations can be parallelized trivially.In the ...