Electronic excitations in semiconductors. II. Application of the theory to diamond

A previously developed theory for calculating electronic excitations in semiconductors is applied to diamond. The starting point is a self-consistent-field calculation within a given basis set of Gaussian-type orbitals. From that the fully correlated electronic wave function and correlation energy are calculated. The reduction of the direct energy gap and the widths of valence and conduction bands due to correlations are studied and compared with experiments. The results can be understood in terms of simple physical pictures. The largest correlation energy contribution results from the polarization cloud of the excited electron and hole pair, but changes in the ground-state correlations due to the presence of the excited electron are also of importance. Intra-atomic correlations such as relaxation effects are only estimated from molecular calculations. A detailed comparison with other methods is made, in particular with the local approximation to the density-functional formalism.