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From Hückel to Clar: a block-localized description of aromatic systems

  • Fabien Barrois , Yannick Carissan , Nicolas Goudard , Denis Hagebaum-Reignier und Stéphane Humbel ORCID logo EMAIL logo
Veröffentlicht/Copyright: 16. Juni 2025

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.


Corresponding author: Stéphane Humbel, Aix Marseille Univ, CNRS, Centrale Med, ISM2, UMR 7313: Institut des Sciences Moléculaires de Marseille 13397, Marseille, Cedex 20, France, e-mail:
Article note: A collection of invited papers to celebrate the UN’s proclamation of 2025 as the International Year of Quantum Science and Technology.

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.

  1. Research ethics: Not applicable.

  2. Informed consent: Not applicable.

  3. 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.

  4. Use of Large Language Models, AI and Machine Learning Tools: To improve language.

  5. Conflict of interest: The author states no conflict of interest.

  6. Research funding: French Research Ministry.

  7. 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 π a b = 1 2 ( a + b ) is bioccupied: | ψ i = | π a b π a b ¯ | . In the block algorithm such a bioccupied π ab is the solution of the isolated {a = b} subsystem and 2 electrons. For a sextet located on atoms af, the subsystem is made of the 6 atomic orbitals connected together to form the ring, solved at the Hückel level, and the lowest orbitals of the subsystem (π 1 π 2 π 3) are filled with 6 electrons. The blocks coexist with the former bond, lone pair and radical definitions (6).

(6) | ψ i = | π 1 π 1 ¯ π 2 π 2 ¯ π 3 π 3 ¯ π a b π a b ¯ |

Singlet coupled radicals are considered with two coupled determinants | a b ¯ | + | b a ¯ | . We allow two blocks of a single electron to be coupled together, but only one pair of coupled electrons are implemented. Besides, an atomic orbital must belong to a single block as it is the case in the Block Localized Wavefunction (BLW) method. 47

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, ψ I | ψ I I = | π 12 π 12 ¯ | | | π 23 π 23 ¯ | .

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 1 t , where t is the total number of permutations in the term.

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 S I , I I = 1 4 , thanks to the orthonormality of the atomic orbitals in the Hückel approximation.

(7) | π a b π a b ¯ | | | π b c π b c ¯ | = π a b π a b ¯ | π b c π b c ¯ = 1 2 ( a + b ) 1 2 ( a + b ¯ ) | 1 2 ( b + c ) 1 2 ( b + c ¯ ) = 1 4 a a ¯ + b b ¯ + a b ¯ + b a ¯ | b b ¯ + c c ¯ + b c ¯ + c b ¯ = 1 4 a | b = 0 a ¯ | b ¯ = 0 + + b | b b ¯ | b ¯ + = 1 4

For larger systems, for instance 4 electron systems, the permutations in the right-hand part writes as in eq. 8.

(8) | a a ¯ b b ¯ | | | c c ¯ d d ¯ | = a a ¯ b b ¯ | c c ¯ d d ¯ a a ¯ b b ¯ | d c ¯ c d ¯ a a ¯ b b ¯ | c d ¯ d c ¯ + a a ¯ b b ¯ | d d ¯ c c ¯ = s a c 2 s b d 2 s a d s a c s b c s b d s a c s a d s b d s b c + s a d 2 s b c 2

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Received: 2025-04-25
Accepted: 2025-05-22
Published Online: 2025-06-16
Published in Print: 2025-09-25

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