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We evaluate the correlation between binding energy (BE) and electron density ρ(r) at the bond critical point for 28 neutral hydrogen bonds, recently reported by Emamian and co-workers (J. Comput. Chem., 2019, 40, 2868). As an efficient tool, we use local stretching force constant kHBa derived from the local vibrational mode theory of Konkoli and Cremer. We compare the physical nature of BE versus kHBa , and provide an important explanation for cases with significant deviation in the BE- kHBa relation as well as in the BE-ρ(r) correlation. We also show that care has to be taken when different hydrogen bond strength measures are compared. The BE is a cumulative hydrogen bond strength measure while kHBa is a local measure of hydrogen bond strength covering different aspects of bonding. A simplified and unified description of hydrogen bonding is not always possible and needs an in-depth understanding of the systems involved.

Using catastrophe theory and the concept of a mutation path, an algorithm is developed that leads to the direct correlation of the normal vibrational modes of two structurally related molecules. The mutation path is defined by weighted incremental changes in mass and geometry of the molecules in question, which are successively applied to mutate a molecule into a structurally related molecule and thus continuously converting their normal vibrational spectra from one into the other. Correlation diagrams are generated that accurately relate the normal vibrational modes to each other by utilizing mode-mode overlap criteria and resolving allowed and avoided crossings of vibrational eigenstates. The limitations of normal mode correlation, however, foster the correlation of local vibrational modes, which offer a novel vibrational measure of similarity. It will be shown how this will open new avenues for chemical studies. © 2017 Wiley Periodicals, Inc.