Theory of the crystal structures of selenium and tellurium: The effect of generalized-gradient corrections to the local-density approximation

The crystal structure and phase stability of selenium and tellurium have been investigated by total-energy calculations within the local-density approximation (LDA), without and including generalized-gradient corrections (GGC). We find that the LDA underestimates the equilibrium volume by as much as 15% and predicts a crystal structure that is much more isotropic than observed: the nearest-neighbor distances in the helical chains are increased, but the interchain distances are reduced. Adding generalized-gradient corrections to the LDA results in a crystal structure, equilibrium volume, and binding energy in better agreement with experiment. We show that the effect of the GGC's is to add a uniform internal pressure to the system, the local bonding properties at constant volume, however, are unchanged.