Electronic energy singularities of weakly H-bonded ammonium dimer

Quantum and molecular mechanics-based electronic energy studies of weak H-bonded ammonium dimer show a distinctive feature in energy profile when computed by different QM methods contrast to MM methods. MM-based MMFF and SYBYL methods show a smoothly varying dihedral energy profile for torsion angle variation around weak N1-H5 held by H-bond strength of around 13 KJ/mol. All the QM-based methods HF, B3LYP and MP2 show noisy and unstable torsion-dependent electronic energy profile for H-bonded ammonium dimer. Exploring energy surface beyond bond length shows singularities and discontinuities. QM-based computation of dipole moment shows several discrete values with jumps and discontinuities with torsion angle variation for ammonium dimer. Also repeated computations and reverse torsion energy profile show a persistent singularity feature observed in all standard QM techniques.