Within a perturbation theory-based framework (Section 5.2), states with opposite parity are coupled whereas states with the same parity are not. The even Γ + 4 conduction band edge in rocksalt SnSe can therefore couple to the secondary conduction band states with odd Γ - 4 and Γ - 5 ⊕ Γ - 6 symmetries (Figure ). Additionally, bands that are closer in energy are coupled more strongly,