Here the diagonal elements are the diabatic energies. The V %& corresponds to the resonance coupling between |A 1 B * ⟩ and |A * B 1 ⟩, which is historically approximated as the Coulombic interaction between monomer transition dipole moments (TDM). The coupling terms t 21 , t 24 , t 31 , t 34 (often called the charge transfer integral) represent the coupling between the LE and CT states, which are usually proportional to the overlap integral of frontier orbitals. Compared with other terms, the coupling between different CT states, V +, , is usually negligible as their wavefunctions differ in two spin orbitals. All monomers and dimers are assumed to obey Kasha's rule, meaning that the fluorescence energy E 5% '. is identical to the energy of the first singlet state (S1) of AB, the smallest eigenvalue of the Hamiltonian matrix.