We wanted to validate the use and transferability of DELFI in two real in-silico experiments. The first one is the choice of a functional to compare the UV/Vis absorption of two isomers that show completely different optical properties, which can be further modulated by different degrees of functionalization. We chose indoline spiropyran (SP) photoswitch as the target molecule , which shows an absorption in the UV region and undergoes photoisomerization to the merocyanine (MC) form absorbing in the visible region. The photoisomerization and the well-defined absorption ranges are exploited in different applications in material and biological sciences , and it can be modulated by changing functional group on both indoline and chromene moieties . It is important to choose a single functional that might satisfactorily describe both molecules with different substitutions to correctly DELFI -November 13, 2023 compare their optical properties. We considered 5 potential substitutions, for both SP and MC, and ran DELFI on all these ten molecules. A pattern of functionals is predicted to be within the first top three performing functionals (1), with ωB97X-V, M11, and LRC-wPBE being the most recurrent ones. For each of the molecules, the three functionals are predicted to perform similarly. Interestingly, differences between the overall performance of TD-DFT according to the substitutions are found, with scores being systematically lower in the presence of the nitro groups. ωB97X-V (74) rcamB3LYP ( ) M11 ( )