Domain-based local pair natural orbital CCSD(T) calculations of fourteen different S2 intermediates for water oxidation in the Kok cycle of OEC of PSII. Re-visit to one LS-two HS model for the S2 state

Domain-based local pair natural orbital (DLPNO) coupled cluster single and double (CCSD) with triple perturbation (T) correction methods were applied for fourteen different S-2 structures of the CaMn4O5 cluster in oxygen evolving complex (OEC) of photosystem II (PSII). The DLPNO-CCSD(T-0) calculations elucidated that the right (R)-opened S-2aYZ structure (a = O2- at the O-(5) site, Y = W2 and Z = W1) with the low spin (LS) (S = 1/2, g = 2) state and two left (L)-opened S(2aYZ )structures with the high spin (HS) (S = 5/2, g = 4; g > 4) state were nearly degenerated in energy, supporting previous one LS-two HS model for the S-2 state in compatible with recent EXAFS and EPR results.