Comparison of chemical bond regulation by external electric fields in different directions: a case study of the H-H bond

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Oriented external electric fields (EEFs) enable tunable regulation of molecular electronic structures and chemical bonds, yet the directional dependence of their modulation on nonpolar covalent bonds remains insufficiently explored. Taking the H–H bond as a model system, this work systematically compares the effects of bond-axis-parallel and perpendicular EEFs (0–0.025 a.u.) using CCSD/aug-cc-pVTZ calculations coupled with multi-dimensional wavefunction analysis. Significant directional anisotropy is revealed. Parallel EEFs drive electron migration along the bond axis, break charge symmetry, elongate bond length and reduce bond order, thus detectably weakening the bond, with atomic charges exhibiting an excellent linear correlation with field strength. In contrast, perpendicular EEFs only induce an overall lateral shift of the electron density distribution without disrupting interatomic charge symmetry, leading to much weaker effects on bond parameters. EEFs in both directions can induce linearly responsive dipole moments with a relatively mild directional difference in polarizability. This study clarifies the directional regulation rule and microscopic mechanism of EEFs on nonpolar covalent bonds, providing theoretical guidance for precise bond manipulation and EEF-assisted catalysis design.