The performance of anisotropic dielectric approach on the excited state of the photosystem II reaction center.

The primary charge separation in Photosystem II (PSII) is governed by a complex interplay between the chromophore cofactors and the electrostatic protein matrix. While the protein environment is known to impose a functional asymmetry favoring the active branch the D1, a quantitative understanding of its dielectric influence on spectroscopic properties remains a challenge. This study employs a state-of-the-art computational approach, combining time-dependent density functional theory (TDDFT) with screened range-separated hybrid (SRSH) functionals, to investigate the excited state energies and character of a PSII reaction center. We use a simplified tetramer model as PD1PD2ChlD1ChlD2 and compare two implicit solvation methods: an isotropic dielectric model via polarizable continuum model (PCM) and a novel anisotropic model PCM(r) via Poisson equation solver (PEqS) that accounts for the heterogeneous dielectric microenvironments of the D1 and D2 branches. Our results demonstrate that the isotropic model captures general environmental screening trends but fails to reproduce the correct energetic ordering of key chromophores, predicting a blue shift for ChlD1 that contradicts the experimental red shift. In contrast, the anisotropic SRSH-PCM(r) model, with a higher dielectric constant assigned to the D1 branch (ɛ1= 3.5-5.0) and a lower one for the D2 branch (ɛ2= 1.5), not only successfully yields bright state energies with correct ordering and their localization but also an electrochromic shift (deviated only 0.03 eV) in qualitative agreement with experimental absorption data. This work establishes that dielectric anisotropy is not merely a minor correction but a critical factor for accurately modeling the spectroscopic properties of PSII, providing a more realistic framework for understanding the role of the protein environment.