  The construction of exponentially localized Wannier functions for a set of
bands requires a choice of Bloch-like functions that span the space of the
bands in question, and are smooth and periodic functions of k in the entire
Brillouin zone. For bands with nontrivial topology, such smooth Bloch functions
can only be chosen such that they do not respect the symmetries that protect
the topology. This symmetry breaking is a necessary, but not sufficient
condition for smoothness, and, in general, finding smooth Bloch functions for
topological bands is a complicated task. We present a generic technique for
finding smooth Bloch functions and constructing exponentially localized Wannier
functions in the presence of nontrivial topology, given that the net Chern
number of the bands in question vanishes. The technique is verified against
known results in the Kane-Mele model. It is then applied to the topological
insulator Bi2Se3, where the topological state is protected by two symmetries:
time reversal and inversion. The resultant exponentially localized Wannier
functions break both these symmetries. Finally, we illustrate how the
calculation of the Chern-Simons orbital magnetoelectric response is facilitated
by the proposed smooth gauge construction.
