HOMOLUMO

HOMOLUMO, often written as HOMO–LUMO, refers to the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a molecule. It is a central concept in molecular orbital theory and quantum chemistry, used to describe the ease with which a molecule can be electronically excited. The gap serves as a simplified descriptor of a molecule’s electronic structure and reactivity.

The size of the HOMOLUMO gap correlates with several electronic properties: larger gaps typically indicate greater chemical stability and lower reactivity, while smaller gaps are associated with higher polarizability and the potential for absorption of visible or near-infrared light. The gap also influences charge transport properties in materials.

Practically, the HOMOLUMO gap can be inferred from the onset of absorption in UV–visible spectroscopy or estimated computationally from quantum chemical calculations. Time-dependent density functional theory (TD-DFT) is commonly used to predict the energy of the first allowed electronic transition, though the raw Kohn–Sham gap from standard DFT methods often underestimates the true excitation energy. Excitonic and solid-state effects can further modify observed transitions in practice.

Factors that influence the gap include the extent of conjugation, aromaticity, ring size, substituents, heteroatoms, charge state, and metal coordination in complexes. In materials science and chemistry, the HOMOLUMO gap guides the design of organic semiconductors, dyes, and photovoltaic materials, as well as molecular electronics and nonlinear optical systems. It is a useful guide, but should be complemented by more detailed treatments of excitations and environment effects.