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rnA simplified LCAO-DFVLDA scheme for calculations of structure and electronic structure of large molecules, clusters, and solids is presented. Forces on the atoms are calculated in a semiempirical way considering the electronic states. The small computational effort of this treatment allows one to perform molecular dynamics (MD) simulations of molecules and clusters up to a few hundred atoms as well as corresponding simulations of condensed systems within the Born-Oppenheimer approximation. The accuracy of the method is illustrated by the results of calculations for a series of small molecules and clusters.

In this paper we propose an extension of the self-consistent charge-density-functional tight-binding ͑SCC-DFTB͒ method ͓M. Elstner et al., Phys. Rev. B 58, 7260 ͑1998͔͒, which allows the calculation of the optical properties of finite systems within time-dependent density-functional response theory ͑TD-DFRT͒. For a test set of small organic molecules low-lying singlet excitation energies are computed in good agreement with first-principles and experimental results. The overall computational cost of this parameter-free method is very low and thus it allows us to examine large systems: we report successful applications to C 60 and the polyacene series.

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The atomic structures, electrical properties, and line energies for threading screw and threading edge dislocations of wurtzite GaN are calculated within the local-density approximation. Both dislocations are electrically inactive with a band gap free from deep levels. These results are understood to arise from relaxed core structures which are similar to (1010) surfaces. [S0031-9007(97)

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A common feature of traditional tight-binding (TB) methods is the non-self-consistent solution of the eigenvalue problem of a Hamiltonian operator, represented in a minimal basis set. These TB schemes have been applied mostly to solid state systems, containing atoms with similar electronegativities. Recently self-consistent TB schemes have been developed which now allow the treatment of systems where a redistribution of charges, and the related detailed charge balance between the atoms, become important as e.g. in biological systems. We discuss the application of such a method, a self-consistent charge density-functional based TB scheme (SCC-DFTB), to biological model compounds. We present recent extensions of the method: (i) The combination of the tight binding scheme with an empirical force field, that makes large scale simulations with several thousand atoms possible. (ii) An extension which allows a quantitative description of weak-bonding interactions in biological systems. The latter include an improved description of hydrogen bonding achieved by extending the basis set and improved molecular stacking interactions achieved by incorporating the dispersion contributions empirically. In applying the method, we present benchmarks for conformational energies, geometries and frequencies of small peptides and compare with ab initio and semiempirical quantum chemistry data. These developments provide a fast and reliable method, which can handle large scale quantum molecular dynamic simulations in biological systems.