Relative energies among S3 intermediates in the photosystem II revealed by DLPNO coupled cluster and hybrid DFT calculations. Possible pathways of water insertion in the S-2 to S-3 transition

DLPNO-CCSD(T-0) calculations coupled with the exact diagonalization of spin Hamiltonian matrix elucidated two right (R)-opened hydroxide (OH-)-inserted structures with the intermediate spin (IS) (S = 3), left (L)-opened OH- and water-inserted structures with IS (S = 3) and left (L)-opened oxo-inserted structures with the high spin (HS) (S = 6) state. Multiple intermediates in the S-3 state revealed by DLPNO-CCSD(T-0) are compatible with the EPR and XFEL results for the S-3 state of photosystem II.