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.
Highlights
Growing interest in nanomechanical properties is reflected in a wide range of research, from the chemistry of synthesizing suitable nanoscale molecules to the physics of manipulating those objects, whether isolated or adsorbed on surfaces and nanostructures
We cite only a few examples that come closer to our investigations: Nanocavities built by atomic assembly at a metal surface with a scanning tunneling microscope (STM) [7,8,9] as well as nanogaps formed with the mechanically controllable break junction technique [10,11,12,13] are both able to hold in place, bind, and subsequently excite a hydrogen molecule (H2) to perform the desired motion, e.g., vibration or rotation
Aside from the very pronounced depression in the case of negative tip charge, where a large attraction after an initial repulsion is seen for decreasing tip distance, a barely noticeable small dip appears in the ground-state energy of positive tip charge between large and small tip separation
Summary
Growing interest in nanomechanical properties is reflected in a wide range of research, from the chemistry of synthesizing suitable nanoscale molecules to the physics of manipulating those objects, whether isolated or adsorbed on surfaces and nanostructures. The interaction between the externally applied electronic current and a trapped molecule can be revealed as inelastic variations In light of these developments, we investigate a system that combines simplicity at an ab initio level with feasibility at the nanoscale: a H2 molecule bridging two STM-like tips, each with an equal point charge. The ground-state energy and the first vibrational excitation are chosen here as the main quantities to probe the response of the hydrogen nuclear motion upon the action of an external static electric field. This system has more general importance beyond the aforementioned examples and their detailed realizations. It is related to realizable subnanoscale mechanical physical arrangements, while the precision gained here is needed to detect or rule out tiny effects that may arise beyond the BO approximation or beyond more macroscopic modeling
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