  Superfluid to Mott-insulator transitions in atomic BEC in optical lattices
are investigated for the case of number of atoms per site larger than one. To
account for mean field repulsion between the atoms in each well, we construct
an orthogonal set of Wannier functions. The resulting hopping amplitude and
on-site interaction may be substantially different from those calculated with
single-atom Wannier functions. As illustrations of the approach we consider
lattices of various dimensionality and different mean occupations. We find that
in three-dimensional optical lattices the correction to the critical lattice
depth is significant to be measured experimentally even for small number of
atoms. Finally, we discuss validity of the single band model.
