 When the highest occupied molecular orbital (HOMO) of one system and the lowest unoccupied molecular orbital (LUMO) of another system have comparable energies and spatial overlap, the electrons can delocalize and sink into a lower energy level.Often, the HOMO of a system is a full σ (bonding) orbital and the LUMO is an empty σ* (antibonding) orbital.This mixing is a stabilizing interaction and has been widely used to explain such phenomena as the anomeric effect.