HOMO, LUMO

Frontier orbitals are derived from molecular orbital theory and are important in describing both the reactivity and electronic properties of organic molecules. These orbitals are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

The HOMO represents the highest-energy molecular orbital that contains electrons. It is primarily involved in electron donation or electron density distribution from one molecule to another during chemical reactions. Molecules with a high-energy HOMO are typically good electron donors.

The LUMO is the lowest-energy empty molecular orbital that can accept electrons. It is important in understanding electron acceptance or electron density acceptance in chemical reactions. Molecules with a low-lying LUMO are often good electron acceptors.

Understanding the relative energy levels and spatial distributions of the HOMO and LUMO helps predict the behaviour of molecules in reactions but also their electronic characteristics, especially when interfaced with other materials such as in organic photovoltaic devices. The energy gap between the HOMO and LUMO is also significant in evaluating whether a molecule will perform effectively as a semiconductor.

In the DiaDEM database, single-molecule, vacuum HOMO and LUMO values are provided, in units of eV, as precomputed properties and are estimated using the Calibrated TDDFT protocol.

1. HOMO: highest occupied molecular orbital

2. LUMO: lowest unoccupied molecular orbital