Aromatic systems have pretty negative NICS values, antiaromatic
systems have strongly positive NICS values, and nonaromatic cyclic systems should have NICS values close to zero [35,42-44].
When the value of the HOMA index is less than zero, the structure is antiaromatic
Therefore, according to Huckels Rule, they should be antiaromatic
Kim, Synthesis, Characterization, and Spectroscopic Analysis of Antiaromatic
Benzofused Metalloporphyrins, Chem.
They're classified as antiaromatic
molecules, which chemical theory predicts will be extremely reactive--in contrast to aromatics, such as benzene, which have unusually stable structures.
The cyclopentadienyl cation is a common textbook example of an antiaromatic
molecule, a molecule so electronically unstable and, therefore, extremely reactive that it should not exist for any length of time.