  Water oxidation at the oxygen evolving complex (OEC) of the photosystem II is
catalyzed by the core cluster CaMn$_4$O$_5$ which was projected to experience
five intermediate states S$\rm_i$ in the Kok's cycle since 1970's. However, the
detailed dynamics of state evolutions still remains unclear, albeit with the
general fact that the process is initiated by the transfer of photoelectrons
with the steady electron donors of the water molecules. Based on density
functional simulations, we find that the spin flips of Manganese atoms between
the consecutive states in the electric polarization field can be used as a
marker to uncover the intricate dynamic evolutions and the underlying dynamics.
The dynamic electron and proton transfers and water insertion and dissociation
are traced to reveal the evolution pathways of S$_0$-S$_3$ with commensurate
spin flips towards the exact spin configuration of the next state. In
particular, the various water insertions and dissociations at coordination
sites of the S$_2$ open and closed cubane isomers are predicted with
constraints on the necessary spin flips. Our study paves a way to uncover the
animated OEC evolutions with the spin flips that await for more experimental
verifications and lays a solid ground for revealing the mechanism of dioxygen
generation via the pending S$_4$ state.
