Abstract
We report a local description of extended aromatic systems using Clar formalism embedded in Valence Bond-like calculations. We disclose a new implementation of our HuLiS (Hückel/Lewis) program that considers blocks of electrons in addition to bonds and lone pairs/radical centers. The method is based on the Hückel approximation for both the empirical hamiltonian and the atomic orbital orthogonality constraint.
Acknowledgments
Wei Wu is gratefully acknowledged for the XMVB programs. The authors acknowledge the French Research Ministry, Aix-Marseille Université (PhD grant for F.B.) for financial support.
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Research ethics: Not applicable.
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Informed consent: Not applicable.
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Author contributions: YC and NG made most of the implementation. FB made most of the calculations. DHR and SH built the project and wrote the paper.
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Use of Large Language Models, AI and Machine Learning Tools: To improve language.
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Conflict of interest: The author states no conflict of interest.
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Research funding: French Research Ministry.
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Data availability: Software availability: HuLiS is available on https://ctomism2.github.io/hulis/hulis_v3_4_0.jar.
Appendix I: Wavefunctions at the Hückel level
In the Hückel approach, the σ skelleton is disregarded, and we use the basis of the atomic p orbitals that make the π system (and π electrons, 1 per carbon atom for instance). Other atoms are of course available and the parameters are the usual Hückel parameters, 68 which can be modified in the interface. In the following, the p atomic orbitals are noted by their atom’s numbering {1, 2, 3}.
A specific localized structure |ψ
i
⟩ is represented as one (or two) Slater determinant(s) using π orbitals that are built on the relevant atoms. For a bond between atoms a and b, an orbital
Singlet coupled radicals are considered with two coupled determinants
Appendix II: Overlaps at the Hückel level
To show how the overlap between structures is computed in the Hückel framework, we take the example, of the two main resonant structures of the allyl cation,

This overlap should involve several permutations, but the result is obtained when the left-hand side determinant is considered as a spin-orbital product,
69
and the permutations that operate in the right-hand side determinant are restricted to those between electrons of the same spin. Each term is signed by
In our simple two-electron case with no pair of the same spin, there is no permutation and the expansion of the overlap is given by eq. 7. It is found that
For larger systems, for instance 4 electron systems, the permutations in the right-hand part writes as in eq. 8.
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Supplementary Material
This article contains supplementary material (https://doi.org/10.1515/pac-2025-0495).
© 2025 IUPAC & De Gruyter
Artikel in diesem Heft
- Frontmatter
- IUPAC Technical Report
- Acid dissociation constants in selected dipolar non-hydrogen-bond-donor solvents (IUPAC Technical Report)
- Preface
- Introduction to the Special Issue of “The International Year of Quantum”
- Review Articles
- Quantum chemistry of molecules in solution. A brief historical perspective
- From Hückel to Clar: a block-localized description of aromatic systems
- Exploring potential energy surfaces
- Unlocking the chemistry facilitated by enzymes that process nucleic acids using quantum mechanical and combined quantum mechanics–molecular mechanics techniques
- Hypothetical heterocyclic carbenes
- Is relativistic quantum chemistry a good theory of everything?
- When theory came first: a review of theoretical chemical predictions ahead of experiments
- Research Articles
- Exploring reaction dynamics involving post-transition state bifurcations based on quantum mechanical ambimodal transition states
- Molecular aromaticity: a quantum phenomenon
- Using topology for understanding your computational results
- The role of ion-pair on the olefin polymerization reactivity of zirconium bis(phenoxy-imine) catalyst: quantum mechanical study and its beyond
- Theoretical insights on the structure and stability of the [C2, H3, P, O] isomeric family
Artikel in diesem Heft
- Frontmatter
- IUPAC Technical Report
- Acid dissociation constants in selected dipolar non-hydrogen-bond-donor solvents (IUPAC Technical Report)
- Preface
- Introduction to the Special Issue of “The International Year of Quantum”
- Review Articles
- Quantum chemistry of molecules in solution. A brief historical perspective
- From Hückel to Clar: a block-localized description of aromatic systems
- Exploring potential energy surfaces
- Unlocking the chemistry facilitated by enzymes that process nucleic acids using quantum mechanical and combined quantum mechanics–molecular mechanics techniques
- Hypothetical heterocyclic carbenes
- Is relativistic quantum chemistry a good theory of everything?
- When theory came first: a review of theoretical chemical predictions ahead of experiments
- Research Articles
- Exploring reaction dynamics involving post-transition state bifurcations based on quantum mechanical ambimodal transition states
- Molecular aromaticity: a quantum phenomenon
- Using topology for understanding your computational results
- The role of ion-pair on the olefin polymerization reactivity of zirconium bis(phenoxy-imine) catalyst: quantum mechanical study and its beyond
- Theoretical insights on the structure and stability of the [C2, H3, P, O] isomeric family