  Landauer's principle places a fundamental lower limit on the work required to
perform a logically irreversible operation. Logically reversible gates provide
a way to avoid these work costs, and also simplify the task of making the
computation as a whole thermodynamically reversible. The inherent reversibility
of mechanical logic gates would make them good candidates for the design of
practical logically reversible computing systems if not for the relatively
large size and mass of such systems. In this paper, we outline the design and
simulation of reversible molecular mechanical logic gates that come close to
the limits of thermodynamic reversibility even under the effects of thermal
noise, and outline associated circuit components from which arbitrary
combinatorial reversible circuits can be constructed and simulated. We
demonstrate that isolated components can be operated in a thermodynamically
reversible manner, and explore the complexities of combining components to
implement more complex computations. Finally, we demonstrate a method to
construct arbitrarily large reversible combinatorial circuits using multiple
external controls and signal boosters with a working half-adder circuit.
