Variable connectivity index as a tool for solving the ‘anti-connectivity’ problem

A modified variable topological index was constructed to account for the combination of positive and negative contributions of atoms or bonds in structure–property–activity relationships. One such property is the response of the flame ionization detector, which increases with the number of carbon atoms in the molecule while the presence of oxygen atoms suppresses the signal. For a selection of organic compounds a single modified variable topological index, which offers important insight into the role of local structural components, produced a root-mean square error of 0.259 for the effective carbon number compared to 0.425 if no negative contributions are allowed.