Algorithm for Locating SCF Solutions Reveals Conceptual Limitations of the Broken-Symmetry Approach Applied to the Oxygen-Evolving Complex

Abstract

Broken-symmetry (BS) calculations of polynuclear open-shell transition-metal complexes are widely used but rely on critical assumptions including the existence and stability of the self-consistent-field (SCF) solutions and the Heisenberg structure of magnetic interactions. We develop a new algorithm for systematic generation and analyses of SCF solutions and apply it to the oxygen-evolving complex (OEC). Several of these solutions are either electronically unstable or exhibit non-Heisenberg character, challenging the conventional interpretation of BS calculations. With homotopy continuation, the DFT solutions continuously transform into the Hartree–Fock solutions through an electron transfer in metal–oxo fragments, explaining a strong functional dependence of BS calculations. The Brueckner coupled-cluster method with double excitations establishes which SCF solutions provide the best reference determinant for correlated wave function treatments. Expansion of the developed methodology further strengthen the understanding of electronic structure in the Kok cycle of the OEC.