Relative energies among proton-shifted S2 isomers in the photosystem II revealed by DLPNO coupled cluster and hybrid DFT calculations. Proton transfer coupled spin transitions of the CaMn4Ox cluster in OEC of PSII

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DLPNO-CCSD(T) calculations coupled with the exact diagonalization of spin Hamiltonian matrix elucidated two right (R)-opened structures with the low spin (LS) (S = 1/2, g = 2) and intermediate spin (IS) (S = 5/2, g > 4) state and two left (L)-opened structures with the IS (S = 5/2, g = 4; S = 7/2, g > 4) and high spin (HS) (S = 13/2) state. Multiple intermediates in the S2 state revealed by DLPNO-CCSD(T) are compatible with the complex EPR, EXAFS, and XFEL results for the S2 state, indicating proton transfer coupled spin transitions of the CaMn4Ox cluster in OEC of PSII.