A set of approximations used to simplify the molecular orbital analysis of conjugated molecules, suggested by Erich Hückel in 1931. In the Hückel theory, the σ‎ orbitals are treated separately from the π‎ orbitals, the shape of the molecule being determined by the σ‎ orbitals. The Hückel theory makes the approximation that interactions between non-neighbouring atoms are taken to be zero. It enables calculations to be made for conjugated molecules. In particular, the delocalization energy of such molecules can be estimated. Hückel explained the stability of benzene associated with aromaticity in this way. The theory can also be used to analyse delocalized bonding in solids. In 1963 Roald Hoffmann proposed an extended Hückel theory in which σ‎ orbitals and π‎ orbitals are not considered separately. Although extended Hückel theory is more complicated, it is a substantial improvement on the original Hückel theory and has been used extensively in both organic and inorganic chemistry.