  While molecular machines play an increasingly significant role in nanoscience
research and applications, there remains a shortage of investigations and
understanding of the molecular gear (cogwheel), which is an indispensable and
fundamental component to drive a larger correlated molecular machine system.
Employing ab initio calculations, we investigate model systems consisting of
molecules adsorbed on metal or graphene surfaces, ranging from very simple
triple-arm gears such as PF3 and NH3 to larger multi-arm gears based on carbon
rings. We explore in detail the transmission of slow rotational motion from one
gear to the next by these relatively simple molecules, so as to isolate and
reveal the mechanisms of the relevant intermolecular interactions. Several
characteristics of molecular gears are discussed, in particular the flexibility
of the arms and the slipping and skipping between interlocking arms of adjacent
gears, which differ from familiar macroscopic rigid gears. The underlying
theoretical concepts suggest strongly that other analogous structures may also
exhibit similar behavior which may inspire future exploration in designing
large correlated molecular machines.
