For example, take azulene ( InChI=1/C10H8/c1-2-5-9-7-4-8-10(9)6-3-1/h1-8H ). All atoms are aromatic, but not all bonds (also PubChem): These things are complex, but the rise of Open Data helps us out, as well as increasing computing power. Peter has been running two rather projects which may help us out: CrystalEye (Nick: no blog?)