Multiorbital

Multiorbital describes systems in which multiple atomic or molecular orbitals contribute significantly to electronic structure, dynamics, or phenomena, rather than a single dominant orbital. This situation arises in molecules with near-degenerate frontier orbitals or in solids where several orbitals per site are energetically active. The term is used to emphasize orbital degrees of freedom alongside spin, charge, and lattice effects.

In chemistry, multiorbital effects appear when several valence or near-degenerate orbitals participate in bonding, bonding excited states, or chemical reactivity. This often requires multireference electronic structure methods, such as complete active space self-consistent field (CASSCF) or configuration interaction, to capture static correlation and Hund’s coupling. Orbital competition can influence reactivity patterns, spectroscopic transitions, and photochemistry in transition metal complexes and organic systems with close-lying orbitals.

In condensed matter physics, multiorbital models extend single-band frameworks by including multiple orbitals per lattice site. The relevant Hamiltonians incorporate intraorbital and interorbital Coulomb interactions, as well as Hund’s exchange coupling and pair hopping terms. Such models help describe materials with rich orbital physics, including transition metal oxides and iron-based superconductors, where orbital differentiation affects magnetism, metal-insulator transitions, and superconductivity. Phenomena of interest include orbital ordering, orbital fluctuations, spin-orbital entanglement, and orbital-selective Mott transitions or Hund’s metal behavior.

Experimentally and computationally, multiorbital systems demand advanced techniques—spectroscopic probes that resolve orbital contributions (e.g., X-ray spectroscopy, ARPES) and many-body methods capable of treating multiple interacting orbitals with strong correlations.