Speaker: Maria Drosou, Technische Universität Darmstadt

A first-principles description of the primary photochemical processes driving Photosynthesis is a grand scientific challenge. These processes involve diverse pigment assemblies embedded in membrane protein complexes. Popular quantum chemical (QM) approaches based on density functional theory face key limitations, including non-transferability, imbalanced description of charge-transfer transitions, and inherent inability of single-reference approaches to describe double excitations. Multireference wavefunction methods can offer the highest level of insight by explicitly describing the wavefunction for each individual state, but they require informed user input and are computationally more intensive than "black-box" TD-DFT.

Here, we present a solid, transparent, and transferable methodological framework for the multireference description of low-lying excited states of protein-embedded chlorin-based pigments relevant to photosynthetic reaction centers and apply this protocol to the description of the reaction center of Photosystem II.[1] For the QM description of the pigment excited states, we use the complete active space self-consistent field (CASSCF) method, incorporating dynamic correlation effects with n-electron valence state perturbation theory (NEVPT2) to obtain accurate excited-state energies. We then integrate this approach with molecular mechanics (MM) in a QM/MM framework to capture electrostatic effects from the protein matrix, which can uniquely diversify otherwise chemically identical pigments.[2] Our protocol extends to describe multichromophoric charge transfer states that define the primary charge separation events in the Reaction Center of Photosystem II. For CASSCF calculations on multichromophoric systems, we employ a dynamic-correlation-assisted approach, namely active space selection by 1st order perturbation theory (ASS1ST).[3] This optimized protocol is transferable to any photoactive embedded pigment system, marking the first step towards large-scale multireference studies of photosynthetic reaction centers and light-harvesting complexes.

[1] M. Drosou, S. Bhattacharjee, D. A. Pantazis, J. Chem. Theory Comput. 2024, 20, 7210.
[2] A. Sirohiwal, D. A. Pantazis, Acc. Chem. Res. 2023, 56, 2921.
[3] A. Khedkar, M. Roemelt, J. Chem. Theory Comput. 2019, 15, 3522.