The behaviour of the total momentum of a system immersed in the Berend
sen thermostat is analyzed. It has been found that under certain condi
tions the kinetic energy of the system can be transformed from interna
l degrees of freedom to the external ones, and conversely. A concrete
example of such a transformation is presented. The results of molecula
r dynamics simulations of the Fe4S4(SH)4 complex coupled to different
external heat baths are discussed.