In the previous section, it is identified that all fragments inducing high oscillator strength have many π conjugation bonds. It is consistent with the fact that many dye molecules have a large portion of the conjugated π-system. The most critical molecular descriptor determining oscillator strength is the fraction of sp 3 carbon atoms. It is in good agreement with the fact that many known fluorophores have π conjugation systems. The number of stereo-centers and the existence of carbon-carbon double bonds are closely related to the fraction of sp 3 carbon atoms. In other words, it implies that a higher sp 3 carbon fraction leads to a low oscillator strength. The 1 κ shape index, the molecular cyclicity, is ranked in the second place. The index is defined as follows: