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Self-consistent Single-band Approximation for Interacting Boson Systems

Traditionally, the single-band approximation for interacting many-body systems is done with pre-determined single-particle Wannier functions, ignoring the dependence of the Wannier function on interaction. We show that the single-band approximation has to be done self-consistently to properly account the interaction effect on the Wannier functions. This self-consistent single-band approximation leads to a nonlinear equation for Wannier functions, which can be recast into a set of nonlinear equations for Bloch functions. These equations are simplified for two special cases, the superfluid regime and deep in the Mott insulator regime. A simple example with double-well potential is used to illustrate our results.