Quantum Signature of Anisotropic Singularities in Hydrogen Bond Breaking of Water Dimer

In all standard force field-based simulations of organic, bio-molecules, polymers; the torsion angle based Hamiltonian has been an indispensable term to set up molecular simulation. Torsion often termed as a dihedral angle, the coordinate assumes a continuous molecular geometry and energy changes for an angle range from 0 to 360 degrees. However, quantum mechanics-based results presented earlier and in this report show electronic energy will have singularities due to molecular geometry criticality, and torsion based electrotonic energy is not a smooth function of (r, theta) due to bond-breaking geometry. Contrast to force field or molecular mechanics based results of geometrical and energy continuity, continuum of geometry under torsion is not feasible as per quantum mechanical electronic energy computations. This feature of electronic energy is readily observed for weakly H-bonded and VDW dimers. Applying the ab initio methods of Hartree-Fock, Density Functional as well as Moller-Plesset, we have reconfirmed the previous general predictions of electronic energy singularities with torsion angle variation around weak H-bond equilibrium and beyond for water dimer. Due to the quantum nature of the weak chemical bond breaking process leading to break-point conditions in molecules, the singularities in electronic energy is observed contrast to molecular mechanics results. These overlooked results of quantum energy singularities can be useful to improve the current bio-molecular force field and reaction chemistry dynamics involving bond-breaking process. Hypothesis tested: https://zenodo.org/records/12730902