The recent discovery that electrons in nanoscale conductors can act like a highly viscous liquid has triggered a surge of activities investigating consequences of this surprising fact. Here we demonstrate that the electronic viscosity has an enormous influence on the operation of a prototypical ac-current-driven nanomotor. The design of this prototype consists of a diatomic molecule immersed in an otherwise homogeneous electron liquid which carries an ac current. The motion of the diatomic is determined by a subtle balance between the current-induced forces and electronic friction. By ab initio time-dependent density-functional simulations we demonstrate that the diatomic performs a continuous rotation provided the amplitude and frequency of the imposed ac current lie within certain zones of stability. Outside these zones the nuclear motion is either irregular or comes to a standstill. The proposed design of the nanomotor is, arguably, the conceptually simplest realization of the idea of a molecular waterwheel sandwiched between conducting leads.

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• Density functional theory
• Methods in transport
• Molecular dynamics