arXiv subjects

Bang C. Huynh

Publications and source records attributed to Bang C. Huynh.

6 recordsLinked to original sources

arXiv2026

Reactivity of Ambident Nucleophiles in Magnetic Fields: a Combined Conceptual DFT and Current-DFT Study

The influence of strong external magnetic fields (up to $0.30\,B_0$) on the electronic structure and reactivity of ambident nucleophiles is investigated using the nitrite and thiocyanate anions as prototypical examples. To capture magnetic-field-induced changes in reactivity, current-density-functional theory (current-DFT) calculations are interpreted through conceptual density-functional theory (conceptual DFT) descriptors, namely global hardness and local softness via Fukui functions, focusing on the evolution of the electronic structure and associated properties with increasing field strength in different orientations. By extending our previous adiabatic treatment of molecules to include higher-spin states, in analogy with earlier work on atoms, and by considering the symmetry properties of relevant quantum-chemical quantities within full magnetic groups using the QSym$^2$ framework, we uncover substantial magnetic-field-induced modulations of both molecular polarity and the shape of the Fukui function. Despite these modulations, the ambident nucleophilicity of both $(\textrm{NO}_2)^-$ and $\textrm{SCN}^-$ is largely preserved, as is the preference for attack by soft electrophiles at the sulfur end of $\textrm{SCN}^-$. At higher field strengths, however, the Fukui functions become increasingly diffuse, reflecting the growing importance of external magnetic interactions relative to internal electrostatic forces. The resulting redistribution of local reactivity not only predicts reaction pathways with unexpected geometries but also raises the possibility that regioselectivity may become progressively less well-defined in the strong-field regime.

physics.chem-ph↗

arXiv2026

Symmetry-adapted generalised normal-ordered coupled-cluster theory for excited states

Ground and excited electronic states in highly symmetric systems typically possess high degrees of spatial degeneracy as a consequence of point-group symmetry. However, many current quantum-chemical methods struggle to accurately describe the strong correlation effects inherently present in these states, thereby precluding the ability to obtain meaningful insights into the electronic structure of the underlying systems. Consequently, many of their important chemical and spectroscopic properties cannot be reliably computed and predicted. In this article, a new theoretical framework is described that unifies the symbolic treatment of non-Abelian symmetry in QSym$^2$ and the recently developed state-specific multi-reference coupled cluster theory termed Generalised Normal Ordered Coupled Cluster (GNOCC) to describe such difficult ground and excited states in a balanced and targeted manner. This is ensured by the ability of QSym$^2$ to exploit symmetry orbits to restore any broken spatial symmetries and generate symmetry-adapted multi-determinantal wavefunctions, as well as the ability of GNOCC to dynamically correlate arbitrary spin eigenfunctions in a size-extensive and spin-free manner. To illustrate the capabilities of this framework, several ground and excited states in three model systems are examined in detail: (i) octahedral $(\textrm{H}_6)^{2+}$, (ii) octahedral $\textrm{H}_6$, and (iii) tetrahedral $\textrm{Li}_4$. The results demonstrate that the proposed method can target both degenerate and non-degenerate states, while delivering improved numerical performance relative to conventional single-reference coupled-cluster approaches.

physics.chem-ph↗

arXiv2024

Symmetry and reactivity of $π$-systems in electric and magnetic fields: a perspective from conceptual DFT

[Abridged] The extension of conceptual DFT to include external fields in chemical systems is utilised to investigate the effects of strong magnetic fields on the electronic charge distribution and its consequences on the reactivity of $π$-systems. Formaldehyde, H$_2$CO, is considered as a prototypical example and current-DFT calculations are used to evaluate the electric dipole moment together with the electron density and the Fukui functions, which provide insight into how H$_2$CO behaves chemically in a magnetic field. In particular, the symmetries of these quantities are analysed based on group, representation, and corepresentation theories using QSym$^2$. This allows us to leverage the simple symmetry constraints on the macroscopic electric dipole moments to make profound predictions on the more nuanced symmetry transformation properties of the microscopic frontier MOs, electron densities, and Fukui functions. This is especially useful for complex-valued MOs in magnetic fields whose detailed symmetry analyses lead us to define the new concepts of modular and phasal symmetry breaking. Through these, the connection between the vanishing constraints on the electric dipole moments and the symmetry of electron densities and Fukui functions can be formalised, and the inability of the magnetic field in all three orientations considered to induce asymmetry with respect to the molecular plane can be understood from a molecular perspective. Furthermore, the detailed forms of the Fukui functions reveal remarkable reversals in the direction of the C=O dipole moment in the presence of a parallel or perpendicular magnetic field, which can be attributed to the mixing between frontier MOs due to their subduced symmetries in magnetic fields. The findings in this work are also discussed in the wider context of a long-standing debate on the possibility to create enantioselectivity by external fields.

physics.chem-ph↗

arXiv2023

QSym$^2$: A Quantum Symbolic Symmetry Analysis Program for Electronic Structure

Symmetry provides a powerful machinery to classify, interpret, and understand quantum-mechanical theories and results. However, most contemporary quantum chemistry packages lack the ability to handle degeneracy and symmetry breaking effects, especially in non-Abelian groups, nor are they able to characterize symmetry in the presence of external magnetic or electric fields. In this article, a program written in Rust entitled QSym$^2$ that makes use of group and representation theories to provide symmetry analysis for a wide range of quantum-chemical calculations is introduced. With its ability to generate character tables symbolically on-the-fly, and by making use of a generic symmetry-orbit-based representation analysis method formulated in this work, QSym$^2$ is able to address all of these shortcomings. To illustrate these capabilities of QSym$^2$, four sets of case studies are examined in detail in this article: (i) high-symmetry $\textrm{C}_{84}\textrm{H}_{64}$, $\textrm{C}_{60}$, and $\textrm{B}_9^-$ to demonstrate the analysis of degenerate molecular orbitals (MOs); (ii) octahedral $\textrm{Fe(CN)}_6^{3-}$ to demonstrate the analysis of symmetry-broken determinants and MOs; (iii) linear hydrogen fluoride in a magnetic field to demonstrate the analysis of magnetic symmetry; and (iv) equilateral $\textrm{H}_3^+$ to demonstrate the analysis of density symmetries.

physics.chem-ph↗

arXiv2022

On Symmetry and the Reality of Holomorphic Hartree--Fock Wavefunctions

The coalescence and disappearance of Hartree--Fock (HF) solutions as the molecular structure varies have been a common source of criticism for the breakdown of the HF approximation to the potential energy surfaces. However, recent developments in holomorphic HF theory show that this disappearing behavior is only a manifestation of the way conventional HF equations prevent solutions from being analytically continued, but it is unclear what factors govern the existence and the locations of these disappearances. In this work, we explore some of these factors from the perspective of spatial symmetry by introducing a classification for symmetry constraints on electronic-structure calculations. This forms a framework for us to systematically investigate several analytic holomorphic HF solutions of a model $\textrm{[H}_4\textrm{]}^{2+}$ system in STO-3G and demonstrate that, under appropriate conditions, spatial symmetry imposes strict requirements on the reality of certain solutions. The implications for self-consistent-field HF search algorithms are then discussed. Throughout this article, the term reality means the quality of a holomorphic HF solution having real molecular orbitals.

physics.chem-ph↗

arXiv2019

Symmetry in Multiple Self-Consistent-Field Solutions of Transition-Metal Complexes

We use a method based on metadynamics to locate multiple low-energy Unrestricted Hartree--Fock (UHF) self-consistent-field (SCF) solutions of two model octahedral $d^1$ and $d^2$ transition-metal complexes, $[\mathrm{MF}_6]^{3-} (\mathrm{M} = \mathrm{Ti}, \mathrm{V})$. By giving a group-theoretical definition of symmetry breaking, we classify these solutions in the framework of representation theory and observe that a number of them break spin or spatial symmetry, if not both. These solutions seem unphysical at first, but we show that they can be used as bases for Non-Orthogonal Configuration Interaction (NOCI) to yield multi-determinantal wavefunctions that have the right symmetry to be assigned to electronic terms. Furthermore, by examining the natural orbitals and occupation numbers of these NOCI wavefunctions, we gain insight into the amount of static correlation that they incorporate. We then investigate the behaviors of the most low-lying UHF and NOCI wavefunctions when the octahedral symmetry of the complexes is lowered and deduce that the symmetry-broken UHF solutions must first have their symmetry restored by NOCI before they can describe any vibronic stabilization effects dictated by the Jahn--Teller theorem.

physics.chem-ph↗