Issue 20, 2024

Resilience of Hund's rule in the chemical space of small organic molecules

Abstract

We embark on a quest to identify small molecules in the chemical space that can potentially violate Hund's rule. Utilizing twelve TDDFT approximations and the ADC(2) many-body method, we report the energies of S1 and T1 excited states of 12 880 closed-shell organic molecules within the bigQM7ω dataset with up to 7 CONF atoms. In this comprehensive dataset, none of the molecules, in their minimum energy geometry, exhibit a negative S1–T1 energy gap at the ADC(2) level while several molecules display values <0.1 eV. The spin-component-scaled double-hybrid method, SCS-PBE-QIDH, demonstrates the best agreement with ADC(2). Yet, at this level, a few molecules with a strained sp3-N center turn out as false-positives with the S1 state lower in energy than T1. We investigate a prototypical cage molecule with an energy gap <−0.2 eV, which a closer examination revealed as another false positive. We conclude that in the chemical space of small closed-shell organic molecules, it is possible to identify geometric and electronic structural features giving rise to S1–T1 degeneracy; still, there is no evidence of a negative gap. We share the dataset generated for this study as a module, to facilitate seamless molecular discovery through data mining.

Graphical abstract: Resilience of Hund's rule in the chemical space of small organic molecules

Supplementary files

Article information

Article type
Communication
Submitted
29 ⴱⵕⴰ 2024
Accepted
03 ⵎⴰⵢ 2024
First published
07 ⵎⴰⵢ 2024

Phys. Chem. Chem. Phys., 2024,26, 14505-14513

Resilience of Hund's rule in the chemical space of small organic molecules

A. Majumdar and R. Ramakrishnan, Phys. Chem. Chem. Phys., 2024, 26, 14505 DOI: 10.1039/D4CP00886C

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