Nanomechanics of a Hydrogen Molecule Suspended between Two Equally Charged Tips

TL;DR

This study uses variational quantum Monte Carlo to analyze a hydrogen molecule between charged tips, revealing quantum effects and directional behaviors that differ from classical models, with potential applications in molecular machinery.

Contribution

It provides a non-Born-Oppenheimer quantum mechanical analysis of a hydrogen molecule in a nanomechanical setup, highlighting charge-dependent directional behaviors.

Findings

Significant deviations from Born-Oppenheimer predictions.

Directional control of molecular axis by tip charge sign.

Potential for switching configurations in molecular machines.

Abstract

Geometric configuration and energy of a hydrogen molecule centered between two point-shaped tips of equal charge are calculated with the variational quantum Monte-Carlo (QMC) method without the restriction of the Born-Oppenheimer (BO) approximation. Ground state nuclear distribution, stability, and low vibrational excitation are investigated. Ground state results predict significant deviations from the BO treatment that is based on a potential energy surface (PES) obtained with the same QMC accuracy. The quantum mechanical distribution of molecular axis direction and bond length at a sub-nanometer level is fundamental for understanding nanomechanical dynamics with embedded hydrogen. Because of the tips' arrangement, cylindrical symmetry yields a uniform azimuthal distribution of the molecular axis vector relative to the tip-tip axis. With approaching tips towards each other, the QMC sampling shows an increasing loss of spherical symmetry with the molecular axis still uniformly distributed over the azimuthal angle but peaked at the tip-tip direction for negative tip charge while peaked at the equatorial plane for positive charge. This directional behavior can be switched between both stable configurations by changing the sign of the tip charge and by controlling the tip-tip distance. This suggests an application in the field of molecular machines.