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In this paper results of calculations of structural and electronic properties of stoichiometric and nonstoichiometric indiumphosphide clusters with up to around 200 atoms are reported. The calculations were carried through using a simplified LCAO-DFT-LDA-TB scheme. We studied finite spherical parts of the zinc blende and wurtzite crystals, which were allowed to relax to the closest local-energy-minimum structure. For both the relaxed and unrelaxed structure, structural and electronic properties were analyzed. This procedure enables us to identify general and size-dependent properties. The properties of our interest include Mulliken populations, density of states, the HOMO−LUMO gap, the relative total energy, the spatial distribution of the frontier orbitals, and the structural relaxations.

The energetics of nickel clusters (Ni_N, N = 2–100) is studied using the embedded-atom method combined with structure optimization through the variable metric/quasi-Newton optimization method. The developed numerical program is fast and enables the determination of the equilibrium configuration of clusters unbiased by additional assumptions on the start geometries, restrictions on the symmetry, or bond lengths. For each cluster size we examine the binding energy per atom and identify the so-called magic clusters, i.e. the clusters of particularly high stability. The results are found to be in good agreement with those of available experimental and theoretical investigations.

In this paper we study the electronic properties of a single doped chain of transpolyacetylene. First, we discuss the different theoretical approaches for studying infinite, periodic, charged chains within parameter-free, electronic-structure calculations. Subsequently, one of these is applied for transpolyacetylene using a density-functional-based method. In particular, the bond-length alternation and the energy gain due to this alternation as a function of added charge are studied. Thereby, it is demonstrated that transpolyacetylene is lacking an electron–hole symmetry. We also explore whether the density-functional calculations predict a too large bond-length alternation when a finite set of points is used in the k-space sampling. Finally, the first-principles results are analysed within a many-body Hubbard-like model Hamiltonian. We find that when extra charge is added to every cell the resulting Hubbard parameters are considerably larger than the ones usually attributed to transpolyacetylene and closer in value to those of the isolated atom. We discuss this result in relation to the importance of screening effects and structural relaxations.

Crystalline cadmium sulfide is a semiconductor for which the wurtzite and zinc blende structures are energetically almost degenerate. Due to quantum-confinement effects, it is possible to tune the optical properties of finite cadmium sulfide clusters by varying their size. Here, we report results of a theoretical study devoted to the properties of stoichiometric CdnSn clusters as a function of their size n. We have optimized the structure, whereby our initial structures are spherical parts of either of the two crystal structures, and we have studied systems with up to almost 200 atoms. The calculations were performed by using a simplified LCAO-DFT-LDA scheme. The results include the structure, electronic energy levels (in particular the frontier orbitals HOMO and LUMO), and stability as a function of size. The results allow for a unique definition of a surface region. The Mulliken populations indicate that the bonds within this region are more ionic than in the bulk. Furthermore, whereas the HOMO is delocalized over major parts of the nanoparticle, the LUMO is a surface state, which confirms recent experimental findings. Finally, the relative stability of the zinc blende and wurtzite structures is strongly dependent on the size of the system, and there is a close connection between the HOMO-LUMO energy gap and stability.

The structural and electronic properties of pyridine, its oligomers, and polypyridine (PPY) as obtained with density functional methods are presented in this work. Among the different exchange-correlation functionals used, B3LYP gives good structural results, whereas B3P88 predicts more accurately the electronic properties. The calculated first excitation energies of pyridine systems are in good agreement with experimental data. The coupling between the monomers in forming oligomers influences the structural and electronic properties of the system significantly. The trans head-to-head dimer is found to be the most stable form and the only one having a planar geometry. The introduction of a head-to-head or tail-to-tail coupling in order to break the regioregularity of a tetramer changes the frontier orbitals and the total energy of the system. The inclusion of a head-to-head coupling in the central units of a tetramer leads to a global stabilization of the system and lowers the HOMO, producing an increase in the first electronic excitation energy. Finally, the electronic properties of infinite PPY are obtained by extrapolations from those of finite oligomers. The calculated ionization potential, electronic affinity, and (π−π)¹ transition are 6.3, 3.4, and 2.9 eV, respectively, in excellent agreement with previous experimental reports. Furthermore, the band structures and density of states of PPY are calculated using a DFT-LMTO method. The calculated density of states is in good qualitative and quantitative agreement with experimental UPS spectrum for this system.

Results of density-functional calculations on cis- and trans-1,4-polybutadiene are presented. The resulting band structures are compared with those of polyethylene and trans-polyacetylene. Charge-density plots of the highest occupied π band show that the results are in accordance with the intuitive concept of localized π bonds. Tight-binding fits of the band structure show that polybutadiene can be regarded as a copolymer between polyethylene and polyacetylene. A detailed study of the single-particle energies of polybutadiene, polyethylene, polyacetylene, and ethylene allows finally a determination of some of the parameters for an extended Hubbard model that can describe the π electrons of extended hydrocarbons. It is shown that the nearest-neighbor Hubbard parameters are small, indicating that further interactions are effectively screened.