  A computationally-efficient method for evaluating friction in molecular
rotary bearings is presented. This method estimates drag from fluctuations in
molecular dynamics simulations via the fluctuation-dissipation theorem. This is
effective even for simulation times short compared to a bearing's energy
damping time and for rotation speeds comparable to or below typical thermal
values. We apply this method to two molecular rotary bearings of similar size
at 300K: previously studied nested (9,9)/(14,14) double-walled carbon nanotubes
and a hypothetical rotary joint consisting of single acetylenic bonds in a
rigid diamondoid housing. The acetylenic joint has a rotational frictional drag
coefficient of $2 \times 10^{-35}\,\mbox{kg m${}^2$/s}$. The friction for the
nested nanotubes is 120 times larger, comparable to values reported by previous
studies. This fluctuation-based method could evaluate dissipation in a variety
of molecular systems with similarly rigid and symmetric bearings.
