where |𝑐 𝑟,𝑖 | is the absolute value of the coefficient of orbital 𝑖 on center 𝑟. When the overlap is zero, the exchange interaction vanishes and Δ𝐸 ST,SCF is zero. Unfortunately, so is the oscillator strength, 𝑓, between the S 1 excited state and the S 0 ground state, which is non-optimal for OLED materials that should emit light. Consequently, there is a trade-off between having a small HOMO-LUMO overlap to reduce the exchange interaction, while still maintaining a sufficient oscillator strength.