Pentalene is an archetypical antiaromatic molecule and the ground-state structure distorts into a C 2h structure with significant bond-length alternation and positive E(S 1 -T 1 ). As shown in Figure (left), the large positive gap correlates with the overlapping HOMO and LUMO. When pentalene is constrained into a double-bond delocalized D 2h structure, the HOMO and LUMO become non-overlapping and consequently negative E(S 1 -T 1 ) is achieved (Figure , center).