  Monte Carlo and spin dynamics simulations have been used to study the dynamic
critical behavior of RbMnF$_3$, treated as a classical Heisenberg
antiferromagnet on a simple cubic lattice. In an attempt to understand the
difference in the value of the dynamic critical exponent $z$ between experiment
and theory, we have used larger lattice sizes than in our previous simulations
to better probe the asymptotic critical region in momentum. We estimate
$z=1.49\pm 0.03$, in good agreement with the renormalization-group theory and
dynamic scaling predictions. In addition, the central peak in the dynamic
structure factor at $T_c$, seen in experiments and previous simulations, but
absent in the renomalization-group and mode-coupling theories, is shown to be
solely in the longitudinal component.
