The story of Clar's goblet dates back to 1972, when Erich Clar envisioned this mysterious hydrocarbon . It contains an equal number of masked and unmasked carbons, suggesting that all πelectrons should be spin-paired (Fig. ). However, it cannot be represented by a Kekulé structure without leaving unpaired electrons. In its resonance structure, two unpaired electrons are always present. According to classic valence bond theory, it is challenging to reconcile how a diradical molecule can have all its π-electrons spin-paired. The nullity (η) is equal to the difference between the numbers of the maximum set of non-adjacent sites and the remaining sites. The dash lines in the disjoint diradical structures indicate that the two radicals can only be distributed on their respective halves in all resonance structures. c, d, DFT calculated SOMO (α), SOMO (β) (c), spin density distribution (d) of Clar's goblet at um06-2x/6-31g(d,p) level of theory. Herein, SOMOs are actually the same as NMBOs. R and Ar represent substituent groups.