  We present a self-consistent method for deriving moment equations for kinetic
models of polymer dynamics. The Doi model [M. Doi, J.Polymer. Sci., Polym.
Phys. Ed. 19, 229 (1981)] of liquid-crystalline polymers with the Onsager
excluded-volume potential is considered as an example. To lowest order, this
method amounts to a simple effective potential different from the Maier-Saupe
form. Analytical results are presented which indicate that this effective
potential provides a better approximation to the Onsager potential than the
Maier-Saupe potential. Corrections to the effective potential are obtained.
