While there are multitudes of molecules in existence which would appear to be antiaromatic on paper, the number of molecules that are antiaromatic in actuality is considerably less. This is compounded by the fact that one cannot typically make derivatives of antiaromatic molecules by adding more antiaromatic hydrocarbon rings, etc. because the molecule typically loses either its planar nature or its conjugated system of π-electrons and becomes nonaromatic.  In this section, only examples of antiaromatic compounds which are non-disputable are included.
Pentalene is an antiaromatic compound which has been well-studied both experimentally and computationally for decades. It is dicyclic, planar and has eight π-electrons, fulfilling the IUPAC definition of antiaromaticity. Pentalene’s dianionic and dicationic states are aromatic, as they follow Hückel’s 4n +2 π-electron rule.