A QTAIM and stress tensor perspective of large‐amplitude motions of the tetrasulfur tetranitride S4N4 molecular graph

The nature of the bonding interactions including the intramolecular S? S interactions in tetrasulfur tetranitride, S₄N₄ are probed by performing very large amplitude vibrations of all of the 18 normal modes of vibration. The QTAIM and stress tensor point properties are ‐then investigated and found to be highly dependent on the mode of vibration. A considerable degree of metallicity ξ( r _b) is found for the S? S and S? N bonding interactions. A unique bonding feature is found for a small amplitude vibration of the most anharmonic mode of this investigation, mode 2, where the S? S bond critical point (BCP) transforms from a closed‐shell S? S BCP to a shared‐shell S? S BCP. We find 17 new unique QTAIM topologies for the molecular graphs corresponding to the 18 modes of vibration along with seven “missing” topologies that are mapped onto a spanning 2‐D Quantum topology Phase Diagram (QTPD). In addition, eleven unique topologies existing on 3‐D QTPDs are found due to the presence of non‐nuclear attractors (NNAs). We use the stress tensor eigenvalues to explain the invariance of the numbers and types of QTAIM critical points. The applicability of both the stiffness S and stress tensor stiffness S_σ are also explored. Two new bond measures are introduced, a polarizability P and the stress tensor polarizability P_σ which are derived from the stiffness S and stress tensor stiffness S_σ, respectively. © 2016 Wiley Periodicals, Inc.