Electronic Energy Singularities of Weakly H-bonded Ammonium Dimer

Background: 

Following set of data has been provided as a proof of general hypothesis for weak H-bonded and also VDW dimers. The methods of quantum mechanics have been applied are Hartree-Fock, Møller-Plesset, Density Functional Theory with state-of-art quantum chemistry software.  And for this specific case results from NH3 dimer has been reported.

[A]. The general hypothesis first proposed in a paper published online in 2020 and in archive in 2021 [1] with explicit mention of hypothesis that torsional electronic energy has singularities for weak H-Bonds contrast to force field results of energy continuum.

[B]. This energy singularities are associated with quantum geometrical criticality of weak H-bonded or VDW dimers. Any QM energy computation method with reasonable basis set must reveal electronic energy singularities unlike smooth force field or molecular mechanics based dihedral energy continuum for weak H-bonded or VDW dimers.  Also for weak H-bonded atoms if separated by a distance beyond bond length; internal rotation must show energy singularities; or equivalently geometry must show criticality feature in all QM energy computations.

[C]. These QM computational results cannot be explained by classical reasoning because of no stearic clashes among atoms;  molecular mechanics/force field  always predict finite energy beyond bond-length. 

Hypothesis Tested: 

All conformational geometry in weak H-bonded or VDW molecules cannot be allowed as per QM contrast to MM even though no stearic clashes of atoms do exist; bond-breaking involves two aspects: <I> sudden jumps in energy <II> breaking of molecular geometry. The energy singularities results are indicative of geometry break-up or in other words bond breaking. 

References: 

[1]  Ali, M Rejwan and Mezei M. 2021. “Observation of Quantum Signature in Rivastigmine Chemical Bond Break-up and Quantum Energetics, Spectral Studies of Anti-Alzheimer Inhibitors.” Journal of Biomolecular Structure and Dynamics 39 (1): 118–28. https://doi.org/10.1080/07391102.2019.1708462.

[2] Ali, Md R. 2022 “Quantum Signature of Anisotropic Singularities in Hydrogen Bond Breaking of Water Dimer” (12th Triennial Congress of the World Association of Theoretical and Computational Chemists (WATOC), Vancouver, Canada)

[3] Ali, M Rejwan 2025, “Electronic Energy Singularities of Weakly H-bonded Ammonium Dimer"       

Journal of Micromechanics and Molecular Physics   https://doi.org/10.1142/S2424913025500079

Preprint:  https://arxiv.org/abs/2504.17107 

Description of Files: 

File Folder Explanation: All the results have been obtained via SPARTAN-20 version software. Both MM and QM methods have been used to compute torsion angle based electronic energy of NH3 dimer. The files have corresponding extension name for methods used as also listed below: 

NH3-dimer_MM: 

1. MMFF

2. SYBYL

 NH3-dimer_QM

1. HF/6-311G*

2. MP2/CC-PVDz

3. B3LYP/6-311G* (Both forward and reverse torsion electronic energy scan)