Electrostatic energies of the singlet (S), triplet (T) and quintet (Q) pair states of two coupled triplet states are derived in a Heider-London type of approximation. The resulting splittings are determined by electron pair exchange and single electron exchange integrals. The latter are, in particular, responsible for the experimentally observed singlet-triplet splitting. © 1982.
Kollmar, C., Sixl, H., Benk, H., Denner, V., & Mahler, G. (1982). Theory of two coupled triplet states - electrostatic energy splittings. Chemical Physics Letters, 87(3), 266–270. https://doi.org/10.1016/0009-2614(82)83139-4