Theoretical Frameworks for Chemical Bonding and Molecular Structure: A Critical Integrative Review of Orbital, Real-Space and Energy-Decomposition Approaches
Riffat Basharat *
Abdul Ahad Azad Memorial Degree College, Bemina, Srinagar, India.
*Author to whom correspondence should be addressed.
Abstract
Chemical bonding is indispensable to chemical explanation but is not represented by a single quantum-mechanical observable. Contemporary theory therefore relies on complementary models that partition wavefunctions, electron densities or molecular energies into chemically interpretable components. This critical narrative review evaluates the conceptual foundations, evidential strengths and limitations of valence-bond and molecular-orbital theories, correlated wavefunction methods, density functional theory, localised-orbital and natural-bond-orbital analyses, bond-order indices, quantum chemical topology, electron-localisation descriptors, noncovalent-interaction visualisation, and fragment- and real-space energy-decomposition schemes. Literature was selected through transparent searches of accessible scholarly indexes and DOI-linked sources, supplemented by citation chaining, with emphasis on foundational theory, methodological validation and applications that expose disagreement among bonding models. The evidence indicates that valence-bond and molecular-orbital descriptions are not competing ontologies in the exact limit but become differently informative under practical truncations. Localised orbitals and donor–acceptor models provide high chemical legibility, whereas real-space approaches offer orbital invariance and direct access to density topology, electron sharing and atomic-energy partitions. Energy-decomposition analyses add mechanistic resolution but remain sensitive to fragment choice, electronic reference states and the sequence of imposed constraints. Difficult cases—including hypervalency, charge-shift bonding, transition-metal donation and back-donation, aromaticity, weak interactions and heavy-element bonding—show that no isolated descriptor reliably establishes bond existence, strength or mechanism. A defensible interpretation should instead triangulate an adequately converged electronic structure with mutually independent orbital, real-space and energetic evidence, while reporting reference, basis-set and functional sensitivity. Emerging entanglement-based and machine-learning approaches may extend bonding analysis, but their chemical meaning remains conditioned by the partition and training level adopted. The most productive theoretical position is therefore disciplined model pluralism: chemical bonds are robust explanatory constructs when claims are matched to the questions, assumptions and invariances of the methods used.
Keywords: Valence-bond theory, molecular-orbital theory, density functional theory, quantum theory of atoms in molecules, electron localisation, energy decomposition analysis, bond order, quantum chemical topology