For 200 nm, all but two GA searches achieved a score of >1.85, which corresponds to wavelength with 28.5 nm of the target value (assuming the oscillator strength is >0.3). So the majority of the searches get reasonably close to the target, but fail to reach the success criterion of 1.99, which corresponds to a wavelength within 7 nm of the target value. The most likely explanation is that it requires a larger change in excitation energy to shift low wavelength excitations. For example, a shift from 207 to 200 nm requires a change in excitation energy of 0.21 eV, compared to 0.05 eV (ca 1 kcal/mol) for a shift from 407 to 400 nm. Thus, absorption wavelengths around 400 nm are easier to fine-tune using relatively modest molecular changes, compared to 200 nm. Conversely, in the case of 600 nm, almost any change to the structure can easily change the excitation wavelength by 7 nm, so it becomes more difficult to hit the target wavelength exactly compared to 400 nm. This underscores the importance of smooth, incremental scoring functions for search efficiency (Figure ). We recognise that some of these molecules may not be stable (e.g. cyclopentadiene groups tend to dimerise) or represent the most stable tautomer. We merely use absorbance as a score that has a complex dependence on the molecular structure.