FMOxFMO: Elucidating Excitonic Interactions in the Fenna–Matthews–Olson Complex with the Fragment Molecular Orbital Method

Abstract

In order to study Förster resonance energy transfer (FRET), the fragment molecular orbital (FMO) method is extended to compute electronic couplings between local excitations via the excited state transition density model, enabling efficient calculations of nonlocal excitations in a large molecular system and overcoming the previous limitation of being able to compute only local excitations. The results of these simple but accurate models are validated against full quantum calculations without fragmentation. The developed method is applied to a very important photosynthetic pigment–protein complex, the Fenna–Matthews–Olson complex (FMOc), that is responsible for the energy transfer from a chlorosome to the reaction center in the green sulfur bacteria. Absorption and circular dichroism spectra of FMOc are simulated, and the role of the molecular environment on the excitations is revealed.

Towards a quantitative description of excitonic couplings in photosynthetic pigment–protein complexes: quantum chemistry driven multiscale approaches

“…The basic approach to computing the important long-range part of excitonic couplings between chromophores is to calculate an excited state of interest in each chromophore and a transition density for an electronic excitation from the ground state to the excited state. 32 The excitonic couplings are obtained from the Coulomb interaction between the transition densities of the chromophores. Other properties derived from the transition density are the transition dipole moment and the atomic transition charges.…”

“…The methodology for calculations in vacuum was developed earlier. 32 In this work, the focus is on incorporating a PCM description of the protein/solvent environment of the pigments.…”

“…The fragment molecular orbital method (FMO) 26,27,28,29 at the TDDFT level 30,31 has been interfaced with FRET in vacuum. 32 Alternatively, there is an FMO method based on CI with single excitations, 33,34 for computing excitonic couplings 35 in vacuum, and a Green's function approach. 36 In this work, FMO-based TDDFT and Hartree-Fock and configuration interaction with singles (HF/CIS) are combined with FRET and the polarizable continuum model (PCM) 37 so that effects of the protein/solvent environment on the excitonic couplings 38,39 can be studied.…”

Computational and data driven molecular material design assisted by low scaling quantum mechanics calculations and machine learning

“…17 For characterizing the electronic structure features in photophysics and photochemistry, fragment-based methods have been successfully applied in accurately calculating excited states in large systems and condensed phases at a high QM level, ranging from simulating optical spectra of aggregationinduced emission (AIE) in molecular crystal materials 18 Olson complex (6853 atoms). 19 In addition, ab initio exciton models can be used for the low-cost study of various excited state processes in complex environments, e.g., optical generation of long-range charge-transfer states in electron donor/ acceptor heterojunctions 20 and singlet ssion in crystalline tetracene. 21 Moreover, novel low-scaling quantum dynamics methods like time-dependent density matrix renormalization group (tDMRG) 22 and multi-conguration time-dependent Hartree (MCTDH) 23 were recently employed for the accurate simulation of real-time nonadiabatic dynamics 24,25 and ultrafast one-or two-dimensional electronic spectroscopy with vibrational resolution 26,27 in complicated systems with a large number of nuclear and electron degrees of freedom, helping build a bridge between theory and experiment in photophysics and photochemistry.…”

“…For characterizing the electronic structure features in photophysics and photochemistry, fragment-based methods have been successfully applied in accurately calculating excited states in large systems and condensed phases at a high QM level, ranging from simulating optical spectra of aggregation-induced emission (AIE) in molecular crystal materials 18 to elucidating excitonic interactions in the Fenna–Matthews–Olson complex (6853 atoms). 19 In addition, ab initio exciton models can be used for the low-cost study of various excited state processes in complex environments, e.g. , optical generation of long-range charge-transfer states in electron donor/acceptor heterojunctions 20 and singlet fission in crystalline tetracene.…”