The Role of High-Level Calculations in the Assignment of the Q-Band Spectra of Chlorophyll

American Institute of Physics
Publication Type:
Conference Proceeding
AIP Conference Proceedings, 2014, (1618), pp. 18 - 22
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We recently established a novel assignment of the visible absorption spectrum of chlorophyll-a that sees the two components Q(x) and Q(y) of the low-energy Q band as being intrinsically mixed by non-adiabatic coupling. This ended 50 years debate as to the nature of the Q bands, with prior discussion poised only in the language of the Born-Oppenheimer and Condon approximations. The new assignment presents significant ramifications for exciton transport and quantum coherence effects in photosystems. Results from state of the art electronic structure calculations have always been used to justify assignments, but quantitative inaccuracies and systematic failures have historically limited usefulness. We examine the role of CAM-B3LYP time-dependent density-functional theory (TD-DFT) and Symmetry Adapted Cluster-Configuration Interaction (SAC-CI) calculations in first showing that all previous assignments were untenable, in justifying the new assignment, in making some extraordinary predictions that were vindicated by the new assignment, and in then identifying small but significant anomalies in the extensive experimental data record.
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