  We have characterized the robust propagation modes of electromagnetic waves
in helical structures by the section Chern number that is defined for a
two-dimensional (2D) section of the three- dimensional (3D) Brillouin zone. The
Weyl point in the photonic bands is associated with a dis- continuous jump of
the section Chern number. A spatially localized Gaussian basis set is used to
calculate the section Chern numbers where we have implemented the
divergence-free condition on each basis function in 3D. The validity of the
bulk-edge correspondence in a 3D photonic crystal is discussed in relation to
the broken inversion symmetry.
