C. Tablero

Global potential energy surfaces for the H3+ system. Analytical representation of the adiabatic ground-state 1 1A′ potential

Adiabatic global potential energy surfaces, for singlet and triplet states of A′ and A″ symmetries, were computed for an extensive grid for a total of 8469 conformations of H3+ system at full configuration interaction ab initio level and using an extended basis set that has also been optimized for excited states. An accurate (root-mean-square error lower than 20 cm−1) global fit to the ground-state potential is obtained using a diatomics-in-molecules approach corrected by several symmetrized three-body terms with a total of 96 linear parameters and 3 nonlinear parameters. This produces an accurate global potential which represents all aspects of ground-state H3+ including the absolute minimum, the avoided crossing and dissociation limits, satisfying the correct symmetry properties of the system. The rovibrational eigenstates have been calculated up to total angular momentum J=20 using hyperspherical coordinates with symmetry adapted basis functions. The infrared spectra thus reproduced is within 1 cm−1 wi...

The lowest triplet state 3A′ of H3 +: Global potential energy surface and vibrational calculations

The adiabatic global potential energy surface of the H3+ system for the lowest triplet excited state of A′ symmetry was computed for an extensive grid of conformations around the minimum region at full configuration interaction ab initio level, using a much more extended basis set than in a previous paper from the same authors. An accurate global fit (rms error lower than 27 cm−1 for energies lower than dissociation into separated atoms and lower than 5 cm−1 for energies lower than the dissociation channel) to these ab initio points and also to part of the previous calculated points (for a total of 7689 energies in the data set) of the lowest triplet excited state of A′ symmetry is obtained using a diatomics-in-molecules approach corrected by one symmetrized three-body term with a total of 109 linear parameters and 1 nonlinear parameter. This produces an accurate global potential which represents all aspects of the bound triplet excited state of H3+ including the minima and dissociation limits, satisfying...

Global fit of ab initio potential energy surfaces I. Triatomic systems

Global potential energy surface (PES) for molecular systems which fit ab initio data can be obtained preserving the accuracy of the ab initio points. The global fitting technique is based in a procedure for triatomic systems including the functional form previously proposed by the authors. The global fit obtained maintains all the symmetry properties of the system including the permutational symmetry and fulfills the stringent criteria needed for molecular dynamical calculations. The program writes out as output file a Fortran-77 program in a form such that the potential and its derivatives with respect to coordinates can be evaluated readily and accurately at arbitrary geometries.

Effects of the orbital self-interaction in both strongly and weakly correlated systems.

The orbital occupation, which is the centerpiece of both self-interaction and several metal-insulator transition analyses, as well as of the local density or generalized gradient approximation with a Hubbard term, is not well defined, in the sense that it is partially ambiguous. A general treatment can be applied to both strongly and weakly correlated systems. When it is applied to an intermediate- and partially filled band within of the host semiconductor gap whose width is less than the semiconductor gap, the original single band can either split as in a Mott transition or not. The former situation is usual and almost always generalized. However the latter also takes place and results from a dilution effect of the self-interaction where a large orbital correlation is reduced if there are other orbital contributions with lower self-interaction in the band. The key is in the choice of the subspace of correlated orbitals. This effect can neither be ignored nor discarded for those systems where there is a substantial mix of states. Examples of these behaviors will be presented and compared to other results. Moreover, the combination of different Hubbard terms acting on different atomic state subspaces can also be used to correct the spurious self-interaction of the bands and the gap underestimation. The relationship between these terms applied to different subspaces of correlated electrons will be presented.

First principles characterization of direct transitions for high efficiency new photovoltaic materials

Some alloys containing a transition metal atom in an III–V host semiconductor show an intermediate half filled band in the middle of the usual semiconductor band gap. The presence of this intermediate band allows to use this material in high efficiency solar cells due to its capability of absorbing low energy photons. In the current work a study of the optoelectronic properties is presented. We mainly focus the work in the obtaining the matrix elements that contribute to direct transitions. We also have analyzed some of the factors on which that process depends. We have also found that some low energy transitions can be found for several points inside the Brillouin zone.

Correlation effects and electronic properties of Cr-substituted SZn with an intermediate band

A study using first principles of the electronic properties of S32Zn31Cr, a material derived from the SZn host semiconductor where a Cr atom has been substituted for each of the 32 Zn atoms, is presented. This material has an intermediate band sandwiched between the valence and conduction bands of the host semiconductor, which in a formal band-theoretic picture is metallic because the Fermi energy is located within the impurity band. The potential technological application of these materials is that when they are used to absorb photons in solar cells, the efficiency increases significantly with respect to the host semiconductor. An analysis of the gaps, bandwidths, density of states, total and orbital charges, and electronic density is carried out. The main effects of the local-density approximation with a Hubbard term corrections are an increase in the bandwidth, a modification of the relative composition of the five d and p transition-metal orbitals, and a splitting of the intermediate band. The results demonstrate that the main contribution to the intermediate band is the Cr atom. For values of U greater than 6 eV, where U is the empirical Hubbard term U parameter, this band is unfolded, thus creating two bands, a full one below the Fermi energy and an empty one above it, i.e., a metal-insulator transition.

Six not-so-easy pieces in intermediate band solar cell research

Abstract. The concept of intermediate band solar cell (IBSC) is, apparently, simple to grasp. However, since the idea was proposed, our understanding has improved and some concepts can now be explained more clearly than when the concept was initially introduced. Clarifying these concepts is important, even if they are well known for the advanced researcher, so that research efforts can be driven in the right direction from the start. The six pieces of this work are: Does a miniband need to be formed when the IBSC is implemented with quantum dots? What are the problems for each of the main practical approaches that exist today? What are the simplest experimental techniques to demonstrate whether an IBSC is working as such or not? What is the issue with the absorption coefficient overlap and the Mott’s transition? What would the best system be, if any?

Mechanism of molecular hydrogen dissociation on gold chains and clusters as model prototypes of nanostructures

The reactivity of H(2) on several gold clusters is studied using density functional theory with generalized gradient approximation methods, as model systems designed to study the main effects determining their catalytic properties under controlled conditions. Border effects are studied in finite linear gold chains of increasing size and compared with the corresponding periodic systems. In these linear chains, the reaction can proceed with no barrier along the minimum energy path, presenting a deep chemisorption well of approximately 1.4 eV. The mechanism presents an important dependence on the initial attacking site of the chain. Linear Au(4) chains joined to model-nanocontacts, formed by 2 or 3 gold atoms, in a planar triangle or in a pyramid, respectively, are also studied. The reaction barriers found in these two cases are approximately 0.24 and 0.16 eV, respectively, corresponding to H(2) attacking the more coordinated edge atom of the linear chain. The study is extended to planar clusters with coordinations IV and VI, for which higher H(2) dissociation barriers are found. However, when the planar gold clusters are folded, and the Au-Au distances elongated, the reactivity increases considerably. This is not due to a change of coordination, but to a larger flexibility of the gold orbitals to form bonds with hydrogen atoms, when the planar sd-hybridization is broken. Finally, it is concluded that the major factor determining the reactivity of gold clusters is not strictly the coordination of gold atoms but their binding structure and some border effects.

Development and implementation of the exact exchange method for semiconductors using a localized basis set

One of the major deficiencies of density functional theory is presented in the approximation of the exchange energy term. An important advance in solving this problem has been the development of orbital-dependent exchange functionals. The exact exchange method is one of the best defined releases of such functionals. Up to now it has been applied in solid systems only using a plane wave representation basis set. In this paper we present a development and implementation of the exact exchange formalism for solid semiconductors using a basis set of localized numerical functions. The implementation of the exact exchange scheme has been carried out in the SIESTA code, as a new path to get the exchange part in the Kohn–Sham energy and potential. This program is an ab initio periodic fully self-consistent density functional code which uses norm-conserving non-local pseudopotentials. Linear combination of confined numerical pseudoatomic orbitals have been used to represent the Kohn–Sham orbitals. The calculation results of the electronic properties of several semiconductor systems using different qualities of the basis set are compared with experimental results and presented in this paper.

Effects of the impurity-host interactions on the nonradiative processes in ZnS:Cr

There is a great deal of controversy about whether the behavior of an intermediate band in the gap of semiconductors is similar or not to the deep-gap levels. It can have significant consequences, for example, on the nonradiative recombination. In order to analyze the behavior of an intermediate band, we have considered the effect of the inward and outward displacements corresponding to breathing and longitudinal modes of Cr-doped ZnS and on the charge density for different processes involved in the nonradiative recombination using first-principles. This metal-doped zinc chalcogenide has a partially filled band within the host semiconductor gap. In contrast to the properties exhibited by deep-gap levels in other systems, we find small variations in the equilibrium configurations, forces, and electronic density around the Cr when the nonradiative recombination mechanisms modify the intermediate band charge. The charge density around the impurity is equilibrated in response to the perturbations in the equil...