title: Influence of Force Fields and Quantum Chemistry Approach on Spectral Densities of BChl a in Solution and in FMO Proteins.
abstract: Studies on light-harvesting (LH) systems have attracted much attention after the finding of long-lived quantum coherences in the exciton dynamics of the Fenna-Matthews-Olson (FMO) complex. In this complex, excitation energy transfer occurs between the bacteriochlorophyll a (BChl a) pigments. Two quantum mechanics/molecular mechanics (QM/MM) studies, each with a different force-field and quantum chemistry approach, reported different excitation energy distributions for the FMO complex. To understand the reasons for these differences in the predicted excitation energies, we have carried out a comparative study between the simulations using the CHARMM and AMBER force field and the Zerner intermediate neglect of differential orbital (ZINDO)/S and time-dependent density functional theory (TDDFT) quantum chemistry methods. The calculations using the CHARMM force field together with ZINDO/S or TDDFT always show a wider spread in the energy distribution compared to those using the AMBER force field. High- or low-energy tails in these energy distributions result in larger values for the spectral density at low frequencies. A detailed study on individual BChl a molecules in solution shows that without the environment, the density of states is the same for both force field sets. Including the environmental point charges, however, the excitation energy distribution gets broader and, depending on the applied methods, also asymmetric. The excitation energy distribution predicted using TDDFT together with the AMBER force field shows a symmetric, Gaussian-like distribution.
authors.affiliation: †Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.
authors.forename: Suryanarayanan
authors.initials: S
authors.lastname: Chandrasekaran
authors.affiliation: †Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.
authors.forename: Mortaza
authors.initials: M
authors.lastname: Aghtar
authors.affiliation: ‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Valleau
authors.affiliation: ‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
authors.forename: Alán
authors.initials: A
authors.lastname: Aspuru-Guzik
authors.affiliation: †Department of Physics and Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kleinekathöfer
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D025541:Bacteriochlorophyll A / Q000737:chemistry*; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry*; D008958:Models, Molecular; D011789:Quantum Theory; D012996:Solutions
chemical_list: D001426:Bacterial Proteins; D025541:Bacteriochlorophyll A; C086320:FMO bacteriochlorophyll protein, Bacteria; D045342:Light-Harvesting Protein Complexes; D012996:Solutions