  In this article, we report a theoretical analysis of a nanoelectromechanical
shuttle based on a multiscale model that combines microscopic electronic
structure data with macroscopic dynamics. The microscopic part utilizes a
(static) density functional description to obtain the energy levels and
orbitals of the shuttling particle together with the forces acting on the
particle. The macroscopic part combines stochastic charge dynamics that
incorporates the microscopically evaluated tunneling rates with a Newtonian
dynamics.
  We have applied the multiscale model to describe the shuttling of a single
copper atom between two gold-like jellium electrodes. We find that energy
spectrum and particle surface interaction greatly influence shuttling dynamics;
in the specific example that we studied the shuttling is found to involve only
charge states Q=0 and Q=+e. The system is found to exhibit two quasi-stable
shuttling modes, a fundamental one and an excited one with a larger amplitude
of mechanical motion, with random transitions between them.
