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In this paper we study the electron–electron correlation in silicon anisotropic quantum dots. A variational approach to the problem of the quantum confinement of two electrons in an ellipsoidal quantum dot is presented. All the calculations are done as a function of the dot aspect ratio, showing how the Coulomb repulsion and dielectric effects (related to the dielectric mismatch between the dot and the surrounding medium) give rise to shape-dependent correlation effects. The energy needed to add a second electron to the ellipsoidal quantum dot is calculated and discussed as well. [ABSTRACT FROM AUTHOR]

By combining ab initio all-electron localized orbital and pseudopotential plane-wave approaches we report on calculations of the electron affinity (EA) and the ionization potential (IP) of (5, 5) and (7, 0) single-wall carbon nanotubes. The role played by finite-size effects and nanotube termination has been analysed by comparing several hydrogen-passivated and not passivated nanotube segments. The dependence of the EA and IP on both the quantum confinement effect, due to the nanotube finite length, and the charge accumulation on the edges, is studied in detail. Also, the EA and IP are compared to the energies of the lowest unoccupied and highest occupied states, respectively, upon increasing the nanotube length. We report a slow convergence with respect to the number of atoms. The effect of nanotube packing in arrays on the electronic properties is eventually elucidated as a function of the intertube distance. [ABSTRACT FROM AUTHOR]

By combining ab initio all-electron localized orbital and pseudopotential plane-wave approaches we report on calculations of the electron affinity (EA) and the ionization potential (IP) of (5, 5) and (7, 0) single-wall carbon nanotubes. The role played by finite-size effects and nanotube termination has been analysed by comparing several hydrogen-passivated and not passivated nanotube segments. The dependence of the EA and IP on both the quantum confinement effect, due to the nanotube finite length, and the charge accumulation on the edges, is studied in detail. Also, the EA and IP are compared to the energies of the lowest unoccupied and highest occupied states, respectively, upon increasing the nanotube length. We report a slow convergence with respect to the number of atoms. The effect of nanotube packing in arrays on the electronic properties is eventually elucidated as a function of the intertube distance.

We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile TiO2 (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first theoretical results on the displacement of the surface atoms in agreement with experiment.

The adsorption of 1-amino-3-cyclopentene on the (100) silicon surface has been studied by methods rooted in the density-functional theory using both delocalized (plane waves, PWs) and localized (Gaussian-type orbitals, GTOs) basis functions. The results obtained by modeling the surface by silicon clusters of different sizes are quite similar, thus confirming that the reaction is quite localized. Furthermore, PW and GTO computations give comparable results, provided that the same density functional and carefully chosen computational parameters (contraction of GTO, pseudopotentials, etc.) are used. Slab computations performed in the PW framework show that the cluster results are retrieved when low-coverage adsorption on the surface is considered. On these grounds, we are quite confident that reaction parameters obtained by the more reliable hybrid density functional (PBE0) are essentially converged, our best estimates of reaction and activation free energies are thus -40 and 6 kcal/mol, respectively.