Beyond SCF Convergence: Robust Electronic-State Assessment of a Biological Dicopper QM Cluster
Abstract
Abstract Accurate electronic-state assignment is essential for reliable quantum-mechanical modeling of multinuclear transition-metal sites, yet self-consistent-field (SCF) convergence alone does not establish physical comparability or orbital stability. Here, we reassess a 95-atom dicopper cluster derived from Manduca sexta prophenoloxidase using a validation strategy based on electron-count consistency, determinant identity, orbital stability, and methodological robustness. Reconstruction of the original calculations showed that two previously compared converged solutions had different total charges, invalidating their apparent energy difference for state ranking. Electron-count-consistent calculations for the Q = + 4 cluster instead yielded broken-symmetry (BS) and high-spin triplet determinants that were effectively degenerate, retained opposite- and parallel-spin localization, respectively, and were independently confirmed as orbital-stable by electronic-Hessian analysis. The BS–triplet splitting remained below 0.4 cm⁻¹ across integration grids, two quantum-chemistry programs, three hybrid functionals, three basis-set levels, and a low-dielectric continuum environment. Although the splitting varied with computational details, no calculation provided a meaningful separation sufficient for an unambiguous ground-state assignment. These results show that electronic-state assessment should precede energetic interpretation and explicitly verify electron count, determinant…
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