  We present a Wannier-based method to calculate the Chern-Simons orbital
magnetoelectric coupling in the framework of first-principles
density-functional theory. In view of recent developments in connection with
strong Z2 topological insulators, we anticipate that the Chern-Simons
contribution to the magnetoelectric coupling could, in special cases, be as
large or larger than the total magnetoelectric coupling in known
magnetoelectrics like Cr2O3. The results of our calculations for the ordinary
magnetoelectrics Cr2O3, BiFeO3 and GdAlO3 confirm that the Chern-Simons
contribution is quite small in these cases. On the other hand, we show that if
the spatial inversion and time-reversal symmetries of the Z2 topological
insulator Bi2Se3 are broken by hand, large induced changes appear in the
Chern-Simons magnetoelectric coupling.
