Shubin Liu

Chemical bonding is traditionally interpreted through orbital-based frameworks such as valence bond and molecular orbital theories. However, density functional theory identifies the electron density—not orbitals—as the fundamental variable, motivating alternative approaches grounded directly in density. This talk presents a unified density-based framework for understanding both covalent bonding and noncovalent interactions. A family of density-derived descriptors—including the Noncovalent Interaction (NCI), Strong Covalent Interaction (SCI), Ultra-Strong Interaction (USI), and Bonding and Nonbonding Interaction (BNI) indices—enables visualization and classification of interactions across the entire strength spectrum, from van der Waals forces and hydrogen bonds to multiple covalent bonds. These tools reveal characteristic spatial signatures in electron density and its derivatives without invoking molecular orbitals. Energetic analyses further show that bond formation arises from a distance-dependent interplay of electrostatic, kinetic, steric, and exchange–correlation contributions. Together, these developments demonstrate that electron density provides a coherent foundation for understanding chemical interactions across molecular systems.