No abstract

The projector augmented-wave ͑PAW͒ method was developed by Blöchl as a method to accurately and efficiently calculate the electronic structure of materials within the framework of density-functional theory. It contains the numerical advantages of pseudopotential calculations while retaining the physics of all-electron calculations, including the correct nodal behavior of the valence-electron wave functions and the ability to include upper core states in addition to valence states in the self-consistent iterations. It uses many of the same ideas developed by Vanderbilt in his ''soft pseudopotential'' formalism and in earlier work by Blöchl in his ''generalized separable potentials,'' and has been successfully demonstrated for several interesting materials. We have developed a version of the PAW formalism for general use in structural and dynamical studies of materials. In the present paper, we investigate the accuracy of this implementation in comparison with corresponding results obtained using pseudopotential and linearized augmented-plane-wave ͑LAPW͒ codes. We present results of calculations for the cohesive energy, equilibrium lattice constant, and bulk modulus for several representative covalent, ionic, and metallic materials including diamond, silicon, SiC, CaF 2 , fcc Ca, and bcc V. With the exception of CaF 2 , for which core-electron polarization effects are important, the structural properties of these materials are represented equally well by the PAW, LAPW, and pseudopotential formalisms. ͓S0163-1829͑97͒00404-9͔

No abstract

Several types of defect structures in PbWO 4 and CaMoO 4 are studied within the framework of density functional theory. While PbWO 4 is currently of greater technological interest, we were able to carry out more extensive calculations for CaMoO 4 , including lattice relaxation, large simulation cells, and more complicated defects. The structural and chemical similarity of the two materials suggests that their defect properties may also be similar. The electronic structure of isolated oxygen vacancies, oxygen and Pb or Ca double vacancies, and substitutional Y are modeled using a supercell approximation. We find that the main effect of oxygen vacancies in PbWO 4 and CaMoO 4 is the introduction of states of W or Mo d character into the band gap. The energies of these defect states are very sensitive to their occupancy. An isolated O vacancy produces a doubly occupied defect state below the conduction band. Removing charge from this defect state lowers its energy and causes additional states of W or Mo d character to move into the band gap. Large supercell simulations for the Ca and O double vacancy in an unrelaxed or slightly relaxed structure produce an unstable electronic structure suggesting the possibility of more extensive lattice distortion. In addition, we also present preliminary results of simulations of interstitial oxygen atoms in CaMoO 4 , finding a relatively stable configuration with the interstitial O forming a weak bond between two MoO 4 clusters.

Calculations of the stopping power (SP) of ion beams in solids have been based on a homogeneous electron gas scattering off a static atom and entail at least one free parameter. Here we report dynamical simulations of ions channeled in silicon. Time-dependent density-functional theory (TDDFT) is used. The calculated SPs are in excellent agreement with the observed oscillatory dependence on atomic number. TDDFT calculations for a homogeneous electron gas demonstrate that both dynamical response and nonuniformities in the electron density are essential to reproduce the data without free parameters.