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The crystal structure of CuS has been confirmed experimentally using the powder diffraction method on the high-resolution powder diffractometer at the Rutherford-Appleton laboratory. The observed crystal structure is P63/mmc. Standard density functional calculations on CuS on a variety of crystal structures are also reported. The calculations also predict P63/mmc as the stable crystal structure. On the basis of the agreement between theory and experiment the authors are able to discuss the details of the bonding in this material.

We present electronic structure and total energy calculations for SiC in a variety of polytype structures using the NRL nonorthogonal tight-binding method. We develop one set of parameters optimized for a combination of electronic and energetic properties using a sp basis, and one optimized for electronic properties using a spd basis. We compute the energies of polytypes with up to 62 atoms per unit cell, and find that the hexagonal wurtzite structure is highest in energy, the 4H structure is lowest in energy, and the cubic zinc-blende structure is in between, in agreement with our linear augmented plane-wave and other calculations. For the sp model we find that the electronic structure of the cubic and hexagonal structures are in good agreement with densityfunctional theory calculations only for the occupied bands. The spd parametrization optimized for the electronic structure of the zinc-blende and wurtzite structures at the equilibrium volume reproduces nearly perfectly both the valence and conduction bands. The sp tight-binding model also yields elastic constants, phonon frequencies, stacking fault energies, and vacancy formation energies for the cubic structure in good agreement with available experimental and theoretical calculations. Using molecular dynamics simulations we compute the finite-temperature thermal expansion coefficient and atomic mean-square displacements in good agreement with available first-principles calculations.

We present a first-principles theory of x-ray Faraday effects. Initially we calculate the difference in absorption rate for right and lefl circularly polarized incident radiation. From this we ax able to define an effective dielectric tensor that can be used to calculate magnetox-ray propetties This work is based on a fully relativistic scattering-theory description of the e l e o n i c stmcture of magnetic materials. The theory is illustrated by a calculation of the Faraday rotaion and induced ellipticity in linearly polarized light incident at the Kedge of iron. Recent experiments compare favourably with the the~ry.

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