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The solid solutions in the system zinc sulfide/zinc selenide (ZnS1-xSe(sub x)) and in the system zinc oxide/zinc sulfide (ZnO1-xSx) are investigated with the help of cyclic cluster model within the semiempirical MSINDO method. The investigation of the mixed crystals ZnS1-xSe(sub x) with the cyclic cluster model shows complete miscibility.

A MinMax self-consistent-field (SCF) approach is derived in the framework of auxiliary density functional theory. It is shown that the SCF convergence can be guided by the fitting coefficients that arise from the variational fitting of the Coulomb potential. An in-core direct inversion of the iterative subspace (DIIS) algorithm is presented. Due to its reduced memory demand this new in-core DIIS method can be applied without overhead to very large systems with tens of thousands of basis and auxiliary functions. Due to the new DIIS error definition systems with fractional occupation numbers can be treated, too. [ABSTRACT FROM AUTHOR]

The development of the cyclic cluster model (CCM) formalism for Kohn-Sham auxiliary density functional theory (KS-ADFT) methods is presented. The CCM is a direct space approach for the calculation of perfect and defective systems under periodic boundary conditions. Translational symmetry is introduced in the CCM by integral weighting. A consistent weighting scheme for all two-center and three-center interactions appearing in the KS-ADFT method is presented. For the first time, an approach for the numerical integration of the exchange-correlation potential within the cyclic cluster formalism is derived. The presented KS-ADFT CCM implementation was applied to covalent periodic systems. The results of cyclic and molecular cluster model (MCM) calculations for trans-polyacetylene, graphene, and diamond are discussed as examples for systems periodic in one, two, and three dimensions, respectively. All structures were optimized. It is shown that the CCM results represent the results of MCM calculations in the limit of infinite molecular clusters. By analyzing the electronic structure, we demonstrate that the symmetry of the corresponding periodic systems is retained in CCM calculations. The obtained geometric and electronic structures are compared with available data from the literature. [ABSTRACT FROM AUTHOR]

Density functional theory optimized basis sets for gradient corrected functionals for 3d transition metal atoms are presented. Double zeta valence polarization and triple zeta valence polarization basis sets are optimized with the PW86 functional. The performance of the newly optimized basis sets is tested in atomic and molecular calculations. Excitation energies of 3d transition metal atoms, as well as electronic configurations, structural parameters, dissociation energies, and harmonic vibrational frequencies of a large number of molecules containing 3d transition metal elements, are presented. The obtained results are compared with available experimental data as well as with other theoretical data from the literature. [ABSTRACT FROM AUTHOR]

The effect of long-range Coulomb interactions on bulk and surface properties calculated within the cyclic cluster model is investigated for the oxides MgO, TiO[sub 2], V[sub 2]O[sub 5], and α-Al[sub 2]O[sub 3]. The Madelung potential is calculated with the Ewald summation method. Results of semiempirical calculations using free clusters, cyclic clusters with and without Madelung potential are compared with each other and with available experimental data from the literature. For the cubic system MgO the effect of long-range interactions on calculated binding energies and structure parameters is small, indicating that the Madelung sum is rapidly converging. On the other hand, the inclusion of lattice sums is of high importance for the simulation of anisotropic systems using the cyclic cluster model. The convergence of calculated properties with increasing cluster size is considerably improved by introducing Madelung sums. Therefore, it is possible to use moderately small cyclic clusters to model the bulk and surface properties of a variety of systems with reasonable accuracy. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]