A novel decomposition of torsional potentials into pairwise interactions: A study of energy second derivatives

A general method of analyzing intramolecular torsional potentials in terms of energy second derivatives that couple the rotating atoms is presented. The method offers a rigorous decomposition of the total torsional potential into pairwise (dihedral) interactions and enables one to derive nonbonded torsional interactions between 1–4 atoms as well as between more distant atoms and sites. The method is demonstrated on ethane, propane and acetaldehyde. It is shown that the 1–4 H…H dihedral potentials in ethane and propane are very similar, thereby supporting the notion of transferable force field potential functions. However, the dihedral potentials that are obtained differ from 1–4 potentials that are used in current force fields. Intramolecular three body effects are clearly seen in this method and are found to be relatively large for the dihedral interactions, although in the one case studied (propane) the overall effect on the methyl‐methyl interaction is negligible due to cancellation of terms. The analysis explicitly shows that the barrier in acetaldehyde is due mainly to the dihedral H…H interaction.