We define the degree of intramolecular CT based on the difference in Mulliken population analysis of the lowest unoccupied NTO (LUNTO), ψ LU N T O , and highest occupied NTO (HONTO), ψ HON T O . This is applicable in INVEST molecules where a mutually exclusive set of atoms participate in the HONTOs and LUNTOs. The HONTO to LUNTO transition represents the best orbital-to-orbital representation from S 0 to S 1 or T 1 .