Failure of Hund’s J in Contemporary DFT+U+J: Insights from Spin-Crossover Fe(II) Complexes
Bibliographic record
Abstract
The effect of the Hund's J terms in the DFT+U+J family of corrective functionals for (semi)local spin-density functional theory is assessed for a series of four octahedrally coordinated Fe(II) spin-crossover molecules spanning the covalent end of the ligand field spectrum. We report values and analyze trends for the Hubbard U and Hund's J parameters determined via minimum-tracking linear response for all valence atomic subspaces and relevant spin states. These parameters are then systematically applied by using simplified rotationally invariant Hubbard functionals to identify the simplest technique capable of yielding reliable adiabatic energy differences with respect to CASPT2/CC benchmarks. Consistent with previous findings, DFT+U improves the underlying DFT electronic density toward the CASPT2 reference, whereas the inclusion of an unlike-spin Hund's J energy term exerts only a marginal additional effect. Notably, the inclusion of this canonical, positively signed Hund's J term fails to remedy, and in fact, exacerbates the deficiencies of DFT+U in describing adiabatic energy differences in strongly covalent molecular systems. It further reveals intrinsic limitations of the contemporary DFT+U+J framework in simultaneously reproducing the charge distribution and energetics of these systems and hints at directions for its improvement.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".