electronicstructure

Electronic structure refers to the arrangement and behavior of electrons in atoms, molecules, and solids. It describes how electrons occupy quantum states, the energy levels involved, and how this distribution governs bonding and reactivity. The ground-state structure is the lowest-energy arrangement; excited states arise when energy is absorbed.

Electrons fill orbitals following the Pauli principle, Hund’s rule, and the Aufbau principle, with spin and electron correlation shaping the distribution. Because nuclei move more slowly, the Born–Oppenheimer approximation separates electronic and nuclear motion. The resulting molecular orbitals or density distributions underpin bonding and properties.

Computational electronic-structure methods include ab initio approaches such as Hartree–Fock and post-Hartree–Fock methods (MP2, CCSD(T)), as well as multiconfigurational methods (CASSCF). Density Functional Theory (DFT) uses electron density and is widely applied, with time-dependent DFT describing excited states. Semi-empirical methods offer cheaper alternatives.

Electronic structure informs chemistry and materials science by predicting bonding, reaction energetics, spectra, and material properties. It is central to interpreting experimental data from spectroscopy and photoelectron techniques and to understanding how electronic changes drive chemical reactivity and optical or electronic behavior.