  We investigate whether open clusters (OCs) tend to energy equipartition, by
means of direct N-body simulations with a broken power-law mass function. We
find that the simulated OCs become strongly mass segregated, but the local
velocity dispersion does not depend on the stellar mass for most of the mass
range: the curve of the velocity dispersion as a function of mass is nearly
flat even after several half-mass relaxation times, regardless of the adopted
stellar evolution recipes and Galactic tidal field model. This result holds
both if we start from virialized King models and if we use clumpy sub-virial
initial conditions. The velocity dispersion of the most massive stars and
stellar remnants tends to be higher than the velocity dispersion of the lighter
stars. This trend is particularly evident in simulations without stellar
evolution. We interpret this result as a consequence of the strong mass
segregation, which leads to Spitzer's instability. Stellar winds delay the
onset of the instability. Our simulations strongly support the result that OCs
do not attain equipartition, for a wide range of initial conditions.
