  A typical nanotribology simulation setup is the semi-infinite substrate,
featuring a sliding bead on top, and with the lower substrate layers
thermostatted to control temperature. A challenge is dealing with phonons that
backreflect from the substrate lower boundary, as these will artificially
reduce the friction FR acting on the sliding bead. One proposed solution is to
use a Langevin thermostat, operating at temperature TLAN, and with the
corresponding damping parameter, GAMMA, optimally tuned such that FR is
maximized [Benassi etal, PHYSREVB, V82, P081401, 2010]. In this paper, the
method is revisited, and related to the substrate phonon lifetime, the
substrate temperature TSUB, and the sliding speed. At low sliding speed, where
the time between stick-slip events is large compared to the phonon lifetime, we
do not observe much dependence of FR on GAMMA, and here thermostat tuning is
not required. At high sliding speed, upon varying GAMMA, we confirm the
aforementioned friction maximum, but also observe a pronounced minimum in TSUB,
which here deviates from TLAN. For substrate particle interactions that are
strongly anharmonic, the variation of FR with GAMMA can be understood as a
manifestation of thermolubricity, backreflections being essentially
unimportant. In contrast, for harmonic interactions, where phonon lifetimes
become very long, FR is strongly affected by backreflecting phonons, though not
enough to overturn thermolubricity.
