Hückel's rule
In keeping with the Möbius-Hückel concept, a cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4"n" + 2 where "n" is a non-negative integer, although clearcut examples are really only established for values of "n" = 0 up to about "n" = 6. Hückel's rule was originally based on calculations using the Hückel method, although it can also be justified by considering a particle in a ring system, by the LCAO method and by the Pariser–Parr–Pople method.