Identification of hydrogen bonds using quantum electrodynamics

A method for the identification of hydrogen bonds was investigated from the viewpoint of the stress tensor density proposed by Tachibana and following other works in this field. Hydrogen bonds are known to exhibit common features with ionic and covalent bonds. In quantum electrodynamics, the covalent bond has been demonstrated to display a spindle structure of the stress tensor density. Importantly, this spindle structure is also seen in the hydrogen bond, although the covalency is considerably weaker than in a typical covalent bond. Distinguishing it from the ionic bond is most imperative for the identification of the hydrogen bond. In the present study, the directionality of the hydrogen bond is investigated as the ionic bond is nearly isotropic, while the hydrogen bond exhibits the directionality. It was demonstrated that the hydrogen bond can be distinguished from the ionic bond using the angle dependence of the largest eigenvalue of the stress tensor density.