Density-functional studies of high-pressure properties and molecular dissociations of halogen molecular crystals

The molecular crystal structure and the molecular dissociation of iodine, bromine, and chlorine under high pressure have been studied using the pseudopotential plane-wave method with both local-density approximated (LDA) and the generalized gradient approximated (GGA) exchange-correlation functionals. Although the generalized gradient approximation overestimates the interlayer distance, it describes the geometry parameters within the molecular plane much better than the local-density approximation. In contrast to the usual geometry optimization for isolated molecules, the local-density approximation largely overestimates the bond lengths of the halogen molecules in the molecular crystal. The GGA dissociation pressures are in good agreement with the experimental values whereas the LDA dissociation pressures are lower than the experimental values. The gap closures for the three elements are observed before the dissociation pressure. The scaled parameters are universal for all three halogens in the molecular phase and at the transition points. Therefore a universal equation of state can be obtained and the dissociation pressures can be well estimated from the scaled volume at ambient pressure.