Computational Investigation of the Spin-Density Asymmetry in Photosynthetic Reaction Center Models from First Principles

We
present a computational analysis of the spin-density asymmetry
in cation radical states of reaction center models from photosystem
I, photosystem II, and bacteria from Synechococcus
elongatus, Thermococcus vulcanus, and Rhodobacter sphaeroides, respectively.
The recently developed frozen-density embedding (FDE)-diab methodology
[J. Chem. Phys., 2018, 148, 214104] allowed us to effectively avoid the spin-density overdelocalization
error characteristic for the standard Kohn–Sham density functional
theory and to reliably calculate spin-density distributions and electronic
couplings for a number of molecular systems ranging from inner pairs
of (bacterio)­chlorophyll a molecules in vacuum to
large proteins including up to about 2000 atoms. The calculated spin
densities show a good agreement with available experimental results
and were used to validate reaction center models reported in the literature.
Here, we demonstrate that the applied theoretical approach is very
sensitive to changes in molecular structures and the relative orientation
of molecules. This makes FDE-diab a valuable tool for electronic structure
calculations of large photosynthetic models effectively complementing
the existing experimental techniques.