arXiv subjects

Duc Anh Lai

Publications and source records attributed to Duc Anh Lai.

4 recordsLinked to original sources

arXiv2026

On the existence of distinct equilibrium configurations under orienting external electric fields

Oriented external electric fields (OEEFs) are ubiquitous in chemistry; however, the effects of fields applied in different directions on molecular systems remain underexplored. A major challenge is that an applied field exerts a torque on a molecule, reorienting the molecular frame and complicating the interpretation of orientation-dependent electric-field effects. Thus, free polar molecules experience orienting rather than oriented fields, such that the field response drives the molecular orientation rather than the other way around. In this work, we explore a new regime of distinct molecular equilibrium configurations in the presence of OEEFs, differing in the relative direction of the external field and the molecular frame. While static dipole moments typically dominate molecular responses to external electric fields, these equilibria are enabled by exploiting molecular polarizability. These distinct "directomers" exhibit unique electronic and nuclear configurations, particularly in their low-lying excited states. We employ oriented electric field vectors referenced to a molecule-fixed principal axis frame along with hybrid analytical-numerical geometry optimization in order to explore the rotational potential energy surface (rPES), as well as a simply analytic model based on equilibrium electrical properties which captures the double-well character of the rPES, including some geometry relaxation effects.

physics.chem-ph↗

arXiv2026

Analytic nuclear gradients including oriented external electric fields in a molecule-fixed frame

Electric field-assisted chemistry has attracted much attention in recent years, particularly in the context of oriented external electric fields for controlling molecular structure and reactivity. Such fields have been explored in a wide range of applications, including switching materials, nanoparticles, controllable catalysts, medicines, and clinical therapies. However, the determination of fixed fields in the laboratory frame becomes ineffective for flexible molecules, as conformational changes can significantly alter the relative orientation between the applied field and molecular structure. In this work, we propose two molecular reference frames -- the principal axis frame and the local reference frame -- to define oriented electric fields within the molecular framework. These coordinate systems powerfully eliminate ambiguities in the relative orientation between the applied field and the molecule. Analytic nuclear gradients in the presence of external electric fields are derived and implemented, with an initial application to field-dependent geometry optimizations of cis- and trans-formanilide. Analysis of the resulting field-induced equilibrium structures reveals distinct structural responses, validating the accuracy and robustness of the proposed formalism. The analytic gradient framework enables systematic investigations of molecular properties and reactivity under arbitrarily oriented electric fields, opening new opportunities for computational modeling and rational design in electric field-controlled chemistry.

physics.chem-ph↗

arXiv2026

A Density-Based Continuous Local Symmetry Measure

Although continuous symmetry theory has attracted increasing attention in modern chemistry, local symmetry remains under-investigated. As a consequence, the relationship between symmetry and chemical behavior is often obscured, limiting the practical use of fuzzy symmetry measures. In this study, we introduce a novel framework for evaluating local symmetry based on electron density localization, and present continuous symmetry representations for several representative molecules. Our approach not only quantitatively captures global symmetry, but also reveals distinctive features of symmetry in a local chemical environment. The related concept, local chirality or chirotopicity, is also discussed. Overall, the proposed local symmetry and chirality measures provide valuable insights into molecular structure and structure-property relationships.

physics.chem-ph↗

arXiv2026

Continuous Local Symmetry: Connection to Reactivity and Recognition

Symmetry is one of the most beautiful yet mysterious concepts in science. In chemical systems, presence of local symmetries at specific fragments often serve as driving forces behind many physicochemical properties, including stability, spectroscopy, and reactivity. Moreover, degree of symmetry varies continuously with molecular dynamics and intermolecular interactions, making it a hidden but decisive factor. In this study, we propose a theoretical framework to quantify continuous degrees of symmetry and chirality localized within constrained regions of a molecular environment. Application of this method to reaction sites of dendralene molecules reveals strong correlations between local symmetry and molecular stability, parity-dependent behavior, and Diels--Alder reactivity. Additionally, representations of local chirality (chirotopicity) fields in porphyrins uncover unique signatures accounting for the chirality recognition power. Overall, these findings highlight the potential of local symmetries within a molecular framework on predicting chemical properties.

physics.chem-ph↗