  A molecular gas system in three dimensions is numerically studied by the
energy conserving molecular dynamics (MD). The autocorrelation functions for
the velocity and the force are computed and the friction coefficient is
estimated. From the comparison with the stochastic dynamics (SD) of a Brownian
particle, it is shown that the force correlation function in MD is different
from the delta-function force correlation in SD in short time scale. However,
as the measurement time scale is increased further, the ensemble equivalence
between the microcanonical MD and the canonical SD is restored. We also discuss
the practical implication of the result.
