Note that the two coupling terms α2 + ζ2 and α2 -ζ2 influence the band curvatures as expressed in Eq. (17). The magnitudes of the coupling terms (i.e. the band-to-band coupling strength) are inversely proportional to the energy separation between the band edges and secondary bands by Eq. ( ); if the energy separation EV -Ei or EC -Ei is small, then the magnitude is larger. In other words, if secondary bands at k0 are close in energy to the band edges, assuming that they possess opposite parities and are coupled, the coupling strength is high. The signs of the coupling terms influence the direction of the band curvatures due to band-to-band coupling. Secondary bands that are below in energy relative to the band edges contribute towards a positive, up-wards curvature, and secondary bands that are above in energy contribute towards a negative, downwards curvature. As a result, the effects of secondary bands on the curvature of band edges can be visualized as a "repulsive force", as illustrated in Figure and demonstrated computationally in Figure .