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Toward Reliable Prediction of the Energy Ladder in Multichromophoric Systems: A Benchmark Study on the FMO Light-Harvesting Complex.

We present an evaluation of the performance of various single-reference QM methods for the prediction of the relative site energies and transition moments of the Q bands in the bacteriochlorophyll a (BChl a) pigments of the Fenna-Matthew-Olson (FMO) complex. We examine the relative merits of ZINDO, CIS, TD-DFT (with the functionals PBE, BLYP, PBE0, B3LYP, and CAM-B3LYP) and RI-CC2 in reproducing the variations across the pigments that occur as a consequence of geometrical and electrostatic effects of the FMO complex by comparison to DFT-BHLYP/MRCI. We find that these pigments are near-multiconfigurational in nature and, thus, constitute critical cases for the RI-CC2 method. The commonly used ZINDO method is fairly reliable for the site energies of the isolated pigments; however, it overestimates the coupling to the environment, thereby leading to variations across the embedded pigments that are too drastic. The overall best performance is provided by the investigated TD-DFT methods, where PBE0 is found to be slightly superior to the other functionals tested.