Many years and great effort have been spent constructing the microscopic model for the room temperature multiferroic BiFeO3 However, earlier models implicitly assumed that the cycloidal wavevector q was confined to one of the three-fold symmetric axis in the hexagonal plane normal to the electric polarization. Because recent measurements indicate that q can be rotated by a magnetic field, it is essential to properly treat the anisotropy that confines q at low fields. We show that the anisotropy energy $-K_3 S^6 sin^6 theta cos 6 phi $ confines the wavevectors q to the three-fold axis $phi =0$ and $+-2 pi/3$ within the hexagonal plane with $theta = pi /2$.
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