Moreover, band-to-band coupling explains why only materials with inverted bands can exhibit the band inversion-driven warping phenomenon. Without band inversion as in PbTe (Figure ) and As2S3 (Figure ), the conduction band edge only feels an upward repulsive force from the opposite-parity valence bands below, resulting in an upward curvature. Similarly, the valence band edge feels a downward repulsive force from the oppositeparity conduction bands above, leading to a downward curvature. The curvature can be reversed (and, therefore, the band edges can be warped) when the parity of the band edge is distinct from that of the secondary bands, which occurs in electronic structures exhibiting inverted bands. In rock-salt SnSe (Figure ), band inversion brings the even parity Γ + 4 Se-p band closer to the secondary Sn-p conduction bands with odd parity (Γ - 4 and Γ - 5 ⊕ Γ - 6 ), resulting in strong coupling between the states. The coupling applies a downward repulsive force from the secondary bands to the Γ + 4 band, leading to a downward curvature of the conduction band edge.