  We perform a systematic study of capture excitation functions by using an
empirical coupled-channel model. In this model, a barrier distribution is used
to take effectively into account the effects of couplings between the relative
motion and intrinsic degrees of freedom. The shape of the barrier distribution
is of an asymmetric Gaussian form. The effect of neutron transfer channels is
also included in the barrier distribution. Based on the interaction potential
between the projectile and the target, empirical formulas are proposed to
determine the parameters of the barrier distribution. Theoretical estimates for
barrier distributions and calculated capture cross sections together with
experimental cross sections of 220 reaction systems with $182 \leqslant Z_{\rm
P}Z_{\rm T} \leqslant 1640$ are tabulated. The results show that our empirical
formulas work quite well in the energy region around the Coulomb barrier. This
model can provide prediction of capture cross sections for the synthesis of
superheavy nuclei as well as valuable information on capture and fusion
dynamics.
