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Calculations of the valence and conduction bands of a covalent crystal by the augmented-plane-wave (APW) method are carried out in the case of diamond for 256 points in the first Brillouin zone. The bands show an unexpected gap between the first conduction bands and those that lie immediately above them. The calculated position in the Brillouin zone of the conduction-band minimum agrees with that determined experimentally by Warren, Wenzel, and Yarnell by the inelastic scattering of neutrons. It proved necessary to use the average potential between the APW spheres as a disposable parameter in order to obtain a satisfactory energy gap for the bands.
R. Keown (Fri,) studied this question.