Noncollinear Spins Provide a Self-Consistent Treatment of the Low-Spin State of a Biomimetic Oxomanganese Synthetic Trimer Inspired by the Oxygen Evolving Complex of Photosystem II

We employ noncollinear density functional theory to show that the low-spin state of Mn3 in a model of the oxygen-evolving complex of photosystem II avoids frustrated spin coupling by adopting a noncollinear arrangement of spins, thereby lowering the energy by 7 kcal/mol. The high-spin state also has noncollinear spins. The optimum self-consistent field solutions for this multinuclear oxomanganese complex correspond to states that cannot be described by the unrestricted Slater determinants used in Kohn–Sham collinear density functional methods. This kind of spin coupling can be important in many open-shell systems, and the conventional collinear spin interpretation of chemical bonding in such systems should be viewed with caution.