We present a generalized projection-based order-N method which is applicable within nonorthogonal basis sets of spatially localized orbitals. The projection to the occupied subspace of a Hamiltonian, performed by means of a Chebyshev-polynomial representation of the density operator, allows the nonvariational computation of band-structure energies, density matrices, and forces for systems with nonvanishing gaps. Furthermore, the explicit application of the density operator to local basis functions gives a powerful method for the calculation of Wannier-like functions without using eigenstates. In this paper, we investigate such functions within models of diamond and fourfold-coordinated amorphous carbon starting from bonding pairs of hybrid orbitals. The resulting Wannier states are exponentially localized and show an ellipsoidal spatial dependence. These results are used to maximize the efficiency of a linear-scaling orthonormalization scheme for truncated Wannier functions. ͓S0163-1829͑98͒01611-7͔
The electronic properties of amorphous carbon structures with varying microscopic mass densities, ranging from 2.0 to 3.52 g/cm, are analyzed. Using a semiempirical density-functional approach the model structures were generated by molecular dynamics performing a simulated cooling of liquid carbon clusters, containing 128 atoms, within periodically arranged cubic supercells. By investigation of properties related to chemical bonding we find a strong control of the band gap by the balance between x bonding and electronic defect generation. As important sources that in8uence the x -vr' band gap the distance distribution between non-o-bonded hybrid orbitals as well as the topological properties of clustering sp (or sp) units are discussed. A number of local densities of states are calculated at representative atoms to validate the classification of x clusters and electronic defect states used.
A semiempirical molecular-dynamic density-functional approach is used to perform a systematic investigation of the stability, structure, and properties of quenched pure and hydrogenated amorphous carbon dependent on the mass density and the hydrogen concentration. By comparing the total structure energies for supercell clusters of equal composition and atom number, we obtain the most stable a-C:H configurations characterized by optimal chemical bonding corresponding to certain mass densities and hydrogen concentrations which control the short-and medium-range order in the material. We present a detailed structural analysis of stable and metastable a-C:H modifications comparing theoretically simulated di8'raction data with electron-scattering results. In addition, we discuss correlations between the atomic structure and the electronic properties which in turn are mediated by the chemical bonding and the clustering of different hybridized atoms.
Results of scanning-tunneling-microscopy (STM) and molecular-dynamics (MD) annealing studies based on quantum-mechanically derived interatomic forces using a semiempirical density-functional approach are combined for analyzing diamond surface structures. Experimentally obtained STM images of diamond (100) and (111)faces on polycrystalline films reveal (1 X 1),(&3 X &3) R 30', and possible (2 X 1) structures. The (100) faces show stable (2 X 1) reconstruction with dimer formation. Surface structures with and without adsorbed hydrogen are determined and their stability is obtained by MD simulated annealing techniques. The bulklike and (&3X&3) 830 structures, as they are observed on grown (111) facets, are attributed to the two diff'erent single atomic (111)layers, which support growth mechanisms, in which the two alternating single atomic layers grow in turn and not simultaneously. The equilibrium surface modifications which have been realized are electronically characterized by investigating the local electronic density of states at selected surface atoms. This information is compared and related to the features seen in the STM images.
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