- Carl-Mikael Suomivuori

*J. Chem. Theory. Comput.* **12**, 2644-2651 (2016).

Publication Date (Web): May 6, 2016

Chlorophylls are light-capturing units found in photosynthetic proteins. We study here the ground and excited state
properties of monomeric, dimeric, and tetrameric models of the special chlorophyll/bacteriochlorophyll (Chl/BChl)
pigment (P) centers P700 and P680/P870 of type I and type II photosystems, respectively.
In the excited state calculations, we study the performance of the algebraic diagrammatic construction through
second-order (ADC(2)) method in combination with the reduced virtual space (RVS) approach and the
recently developed Laplace-transformed scaled-opposite-spin (LT-SOS) algorithm, which allows us,
for the first time, to address multimeric effects at correlated ab initio levels using large basis sets.
At the LT-SOS-RVS-ADC(2)/def2-TZVP level, we obtain vertical excitation energies (VEEs) of 2.00–2.07 and 1.52–1.62 eV
for the P680/P700 and the P870 pigment models, respectively, which agree well with the experimental absorption maxima
of 1.82, 1.77, and 1.43 eV for P680, P700, and P870, respectively.
In the P680/P870 models, we find that the photoexcitation leads to a π → π^{*} transition in which
the exciton is delocalized between the adjacent Chl/BChl molecules of the central pair, whereas the exciton is localized
to a single chlorophyll molecule in the P700 model.
Consistent with experiments, the calculated excitonic splittings between the central pairs of P680, P700, and P870 models are
80, 200, and 400 cm^{1}, respectively.
The calculations show that the electron affinity of the radical cation of the P680 model is 0.4 V larger than for the P870 model
and 0.2 V larger than for P700.
The chromophore stacking interaction is found to strongly influence the electron localization properties of the light-absorbing
pigments, which may help to elucidate mechanistic details of the charge separation process in type I and type II photosystems.

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