where 𝐾 is the exchange integral between the two orbitals involved in the single excitation (normally the HOMO and the LUMO). As defined here, a positive Δ𝐸 ST corresponds to the usual situation with the triplet being lower in energy than the singlet, while a negative Δ𝐸 ST corresponds to an inverted gap. The exchange integral depends strongly on the spatial overlap between the the two orbitals, which in the ZDO approximation can be computed as: