Understanding Primary Charge Separation and Revealing Fermi’s Golden Rule in the Heliobacterium Modesticaldum Reaction Center

We investigate the spectroscopic properties and charge transfer (CT) processes in the homodimeric reaction center of Heliobacterium modesticaldum (HbRC), for which we report the first-principles calculation of Fermi's golden rule (FGR) for the first time in the literature. HbRC is the simplest known analogue to other photosystems and consists of the SPP molecule, the adjacent Acc, and key amino acid residues (His537 and Gln458), along with two water molecules. We identify intrapair CT processes within the SPP molecule, which acts as the donor for the simplified model used in this study. We also examine the SPP⁺Acc^- CT processes, where we demonstrate, in agreement with experimental findings, symmetric hole-electron delocalization across both Acc monomers. The calculated forward CT states are lower in energy than the bright states, facilitating electron transfer through a downhill CT reaction pathway. To explore the influence of surrounding molecular pigments on the electronic structure and FGR rate, we use augmented models incorporating nonsymmetrized molecular orientations. Most FGR rates are found to be on the order of picoseconds (ps). All electronic parameters are derived using an optimally tuned screened range-separated hybrid functional within a polarizable continuum model (SRSH-PCM), considering different dielectric constants.