Singularities in the behavior of density functionals in predictions of singlet biradicals: The 1,2-dichalcogenins
Bibliographic record
Abstract
The performance of conventional exchange-correlation functionals for the description of singlet biradicals which possess a small admixture of an auxiliary electronic state was examined using a broken symmetry spin unrestricted density functional approach. A pure density functional, BLYP, and the hybrid B3LYP and BHandHLYP exchange-correlation functionals were employed to study the thermal ring opening of 1,2-dichalcogenins and the subsequent formation of the bicyclic products. A stepwise mechanism was predicted for the reaction of the 1,2-dithiin and 2-selenathiin rings and no biradical structures were located. For 1,2-diselenin, the biradical stepwise mechanism competes energetically with a concerted reaction which is slightly favored. For 1,2-ditellurin, a high-energy open intermediate with considerable biradical character was located. The hybrid functionals show an increase in biradical character with an increase in the percentage of Hartree–Fock exchange incorporated into the exchange functional. The instability of the restricted Kohn–Sham solutions was used to indicate the existence of biradical structures with lower energy. Interesting incorrect behavior of certain of the hybrid functionals was noted. The instability of the restricted BHandHLYP solution results from too large a contribution of Hartree–Fock exchange. An unexpected stability of the restricted B3LYP solution was found and attributed to cancellation of two different sources of instability: one from the pure density functional and the other from the inclusion of Hartree–Fock exchange.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.004 | 0.005 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".