In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. It was formulated by physical chemist Erich Hückel in 1931. A cyclic ring molecule follows Hückel's rule when the number of its pi electrons equals 4n + 2 where n is zero or any positive integer.
Hückel's rule is not valid for many compounds containing more than three fused aromatic nuclei in a cyclic fashion like in pyrene or coronene.
The Pariser-Parr-Pople method is a more precise method of estimating whether a cyclic ring molecule is aromatic.