  We show that one can properly take into account of the interaction effects
and construct a set of orthonormal Wannier functions for a Bose-Einstein
condensate in an optical lattice. These interaction-dependent Wannier functions
are used to compute the tunneling rate $J$ and the on-site repulsion $U$ in the
Bose-Hubbard model. Both parameters are found to be substantially different
from ones calculated with the single-particle Wannier functions. Our numerical
results of $U$ are found in good agreement with the measured on-site energy in
a recent experiment [Campbell {\it et al.} Science {\bf 314}, 281 (2006)].
