https://mazziotti.uchicago.edu/qcdictionary/f_terms/fundamental_bandgap/index.html The fundamental band gap is the energy difference between the highest energy state of the valence band and the lowest energy state of the conduction band. The valence band is the furthest band from the atomic nucleus that is still occupied by electrons, while the conduction band is a higher-energy range of electronic states where electrons move freely. Hugo en-us