Consequently, correlations between optical modulations and computationally determined eHOMO levels were examined for various subsets of the screened analytes. Here too, a robust correlation was observed between optical responses and HOMO levels for the 18 compound sub-library (fig. ), as well as various other subsets (fig. ) and the entire library of compounds (fig. ). Importantly, while correlation between HOMO and optical responses was strong, high eHOMO levels do not guarantee an optical response (fig. ). Consistent with our earlier heuristic description, these results show that at least two vicinal hydrogen bond donating groups are necessary for optical modulation in addition to the observed correlations with electron densities on the aryl group (fig. ). In most cases, these groups are ortho-to each other but they can be connected through extended conjugation as well (e.g., 4, fig. ).