Exchange energies for direct excitons

A simplified method, based on the model proposed by Baldereschi and Lipari (1971) for investigating the energy levels of direct excitons, is used to calculate the exchange energies to first order. Contributions from second-order intra-series and inter-series terms are considered and found to be negligible for most of the materials investigated. The exchange energies so obtained, are generally in good agreement with existing theoretical and experimental values.