  The hydrodynamic radius of a polymer chain, obtained using Brownian dynamics
simulations of the continuum Edwards model, is found to obey a crossover in the
excluded volume parameter z, which is significantly different from that
observed for the radius of gyration. It is shown that this difference arises
from contributions due to dynamic correlations to the diffusivity, which are
ignored in the commonly used definition of hydrodynamic radius based on the
Kirkwood expression. The swelling of the hydrodynamic radius from the
theta-state, obtained from simulations, shows remarkable agreement with
experimental measurements.
