TIME-DEPENDENT DENSITY FUNCTIONAL STUDY OF MULTIELECTRON TRANSFER IN Cq+-Na-4 COLLISIONS

The process of multielectron transfer from a Na-4 cluster induced by highly charged C6+, C4+, C2+ and C+ ions is studied using the method of time-dependent density functional theory within the local density approximation combined with the use of pseudopotential. The evolution of dipole moment changes and emitted electrons in Na-4 isobtained and the time-dependent probabilities with various charges are deduced. It is shown that the Na-4 cluster is strongly ionized by C6+ and that the number of emitted electrons per atom of Na-4 is larger than that of Na-2 under the same condition. One can find that the detailed information of the emitted electrons from Na-4 is different from the same from Na-2, which is possibly related to the difference in structure between the two clusters.

Structures and elastic properties of OsN2 investigated via first-principles density functional calculations

The structure, elastic, and electronic properties of OsN2 at various space groups: cubic Fm-3m, Pa-3, and orthorhombic Pnnm were studied by first-principles calculations based on density functional theory. Our calculation indicates that the structure in orthorhombic Pnnm phase is energetically more stable compared with cubic systems. It is metallic, mechanically stable and contains diatomic N-N units with the bond distance 1.418 A. These characters are consistent with experimental facts that OsN2 is orthorhombic and metallic. The calculated bulk modulus 394 GPa is also the highest among the considered space groups, slightly larger than previous value 358 GPa. The calculated elastic anisotropic factors and directional bulk modulus showed that OsN2 possess high elastic anisotropy.

Structural manifestation of the delocalization error of density functional approximations: C(4N+2) rings and C(20) bowl, cage, and ring isomers.

The ground state structure of C(4N+2) rings is believed to exhibit a geometric transition from angle alternation (N < or = 2) to bond alternation (N > 2). All previous density functional theory (DFT) studies on these molecules have failed to reproduce this behavior by predicting either that the transition occurs at too large a ring size, or that the transition leads to a higher symmetry cumulene. Employing the recently proposed perspective of delocalization error within DFT we rationalize this failure of common density functional approximations (DFAs) and present calculations with the rCAM-B3LYP exchange-correlation functional that show an angle-to-bond-alternation transition between C(10) and C(14). The behavior exemplified here manifests itself more generally as the well known tendency of DFAs to bias toward delocalized electron distributions as favored by Huckel aromaticity, of which the C(4N+2) rings provide a quintessential example. Additional examples are the relative energies of the C(20) bowl, cage, and ring isomers; we show that the results from functionals with minimal delocalization error are in good agreement with CCSD(T) results, in contrast to other commonly used DFAs. An unbiased DFT treatment of electron delocalization is a key for reliable prediction of relative stability and hence the structures of complex molecules where many structure stabilization mechanisms exist.

Large-scale density functional calculations of the surface properties of the Wigner crystal

The surface properties of the jellium model have been investigated by large supercell computations in the density functional theory-local spin-density (DFT-LSD) approach for planar slabs with up to 1000 electrons. A wide interval of densities has been explored, extending into the stability range of the Wigner crystal. Most computations have been carried out on nominally paramagnetic samples with an equal number of spin-up and spin-down electrons. The results show that within DFT-LSD spontaneous spin polarization and charge localization start nearly simultaneously at the surface for r(s) similar to 20, then, with decreasing density, they progress toward the center of the slab. Electrons are fully localized and spin polarized at r(s) = 30. At this density the charge distribution is the superposition of disjoint charge blobs, each corresponding to one electron. The distribution of blobs displays both regularities and disorder, the first being represented by well-defined planes and simple in-plane geometries, and the latter by a variety of surface defects. The surface energy, surface dipole, electric polarisability, and magnetization pattern have been determined as a function of density. All these quantities display characteristic anomalies at the density of the localization transition. The analysis of the low-frequency electric conductivity shows that in the fluid paramagnetic regime the in-plane current preferentially flows in the central region of the slab and the two spin channels are equally conducting. In the charge localized, spin-polarized regime, conductivity is primarily a surface effect, and an apparent asymmetry is observed in the two spin currents.

Dispersion interactions in room-temperature ionic liquids: Results from a non-empirical density functional

The role of dispersion or van de Waals (VDW) interactions in imidazolium-based room-temperature ionic liquids is studied within the framework of density functional theory, using a recently developed non-empirical functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)], as efficiently implemented in the SIESTA code [G. Roman-Perez and J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009)]. We present results for the equilibrium structure and lattice parameters of several crystalline phases, finding a general improvement with respect to both the local density (LDA) and the generalized gradient approximations (GGA). Similar to other systems characterized by VDW bonding, such as rare gas and benzene dimers as well as solid argon, equilibrium distances and volumes are consistently overestimated by approximate to 7%, compared to -11% within LDA and 11% within GGA. The intramolecular geometries are retained, while the intermolecular distances and orientations are significantly improved relative to LDA and GGA. The quality is superior to that achieved with tailor-made empirical VDW corrections ad hoc [M. G. Del Popolo, C. Pinilla, and P. Ballone, J. Chem. Phys. 126, 144705 (2007)]. We also analyse the performance of an optimized version of this non-empirical functional, where the screening properties of the exchange have been tuned to reproduce high-level quantum chemical calculations [J. Klimes, D. Bowler, and A. Michaelides, J. Phys.: Condens. Matter 22, 074203 (2010)]. The results for solids are even better with volumes and geometries reproduced within 2% of experimental data. We provide some insight into the issue of polymorphism of [bmim][Cl] crystals, and we present results for the geometry and energetics of [bmim][Tf] and [mmim][Cl] neutral and charged clusters, which validate the use of empirical force fields. (C) 2011 American Institute of Physics. [doi:10.1063/1.3652897]

Density Functional study of the photoactive yellow protein's chromophore

We have investigated the structural and electronic properties of p-coumaric acid, the chromophore of the photoactive yellow protein (PYP), by means of first-principles molecular dynamics based on density functional theory (DFT). We have studied the chromophore both in the vacuum and in an extended model which includes the nearest residues in the binding pocket of PYP, as derived from crystallographic data. We have characterized the ground state of the isolated chromophore in its protonated and deprotonated forms and computed the energy barrier involved in the trans to cis isomerization process around the carbon-carbon double bond. A comparison of the optimized structures of the chromophore in the vacuum and in the extended protein model, both in the trans (ground state of PYP in the dark) and cis (first light-activated intermediate) configuration, shows how the protein environment affects the chromophore in the first step of the photocycle. Our model gives an energy storage of 25 kcal/mol associated with the trans-to-cia photoisomerization. Finally, we have elucidated the nature of the electronic excitation relevant for the photochemistry of PYP by means of time-dependent DFT calculations.

Mechanistic Study of 1,3-Butadiene Formation in Acetylene Hydrogenation over the Pd-Based Catalysts Using Density Functional Calculations

Green oil, which leads to the deactivation of the catalysts used for the selective hydrogenation of acetylene, has long been observed but its formation mechanism is not fully understood. In this work, the formation of 1,3-butadiene, known to be the precursor of green oil, on both Pd(111) and Pd(211) surfaces is examined using density functional theory calculations. The pathways containing C-2 + C-2 coupling reactions as well as the corresponding hydrogenation reactions are studied in detail. Three pathways for 1,3-butadiene production, namely coupling plus hydrogenation and further hydrogenation, hydrogenation plus coupling plus hydrogenation, and a two step hydrogenation followed by coupling, are determined. By comparing the effective barriers, we identify the favored pathway on both surfaces. A general understanding toward the deactivation process of the industrial catalysts is also provided. In addition, the effects of the formation of subsurface carbon atoms as well as the Ag alloying on the 1,3-butadiene formation on Pd-based catalysts are also investigated and compared with experimental results.

Density functional study of the C atom adsorption on the α-Fe 2O3 (001) surface

The adsorption of C atoms on the α-Fe₂O₃ (001) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the PBE (Perdew, Burke and Ernzerhof) generalized gradient approximation (GGA) with a plane wave basis set. Upon the optimization on different adsorption sites with coverage of 1/20 and 1/5 ML, it was found that the adsorption of C atoms on the α-Fe ₂O₃ (001) surface was chemical adsorption. The coverage can affect the adsorption behavior greatly. Under low coverage, the most stable adsorption geometry lied on the bridged site with the adsorption energy of about 3.22 eV; however, under high coverage, it located at the top site with the energy change of 8.79 eV. Strong chemical reaction has occurred between the C and O atoms at this site. The density of states and population analysis showed that the s, p orbitals of C and p orbital of O give the most contribution to the adsorption bonding. During the adsorption process, O atom shares the electrons with C, and C can only affect the outermost and subsurface layers of α-Fe₂O₃; the third layer can not be affected obviously. Copyright © 2008 Chinese Journal of Structural Chemistry.

Density Functional Study on Reaction Activity of Steam Decomposition Products Adsorbed on Carbonaceous Surface

A systematic theoretical study on the adsorption of steam and its thermal decomposition products on carbon both zigzag and armchair surface was performed to provide molecular-level understanding of the reaction activity of all these reactants in biomass steam gasification process. All the calculations were carried out using density functional theory (DFT) at the B3LYP/6-31+g(d,p) level. The structures of carbonaceous surfaces, all reactants and surface complexes were optimized and characterized. Based on the value of adsorption heat been obtained from the calculation, the activity of all reactants can be ordered as: O > O2 >H2 >H >OH >H2O for both zigzag and armchair surface, and the adsorption style is physisorption to water molecule and chemisorption to the other dissociated components.

Current-density-functional approach to large quantum dots in intense magnetic fields

Within current-density-functional theory, we have studied a quantum dot made of 210 electrons confined in a disk geometry. The ground state of this large dot exhibits some features as a function of the magnetic field (Beta) that can be attributed in a clear way to the formation of compressible and incompressible states of the system. The orbital and spin angular momenta, the total energy, ionization and electron chemical potentials of the ground state, as well as the frequencies of far-infrared edge modes are calculated as a function of Beta, and compared with available experimental and theoretical results.

Density-functional calculations of magnetoplasmons in quantum rings

We have studied the structure and dipole charge-density response of nanorings as a function of the magnetic field using local-spin-density-functional theory. Two small rings consisting of 12 and 22 electrons confined by a positively charged background are used to represent the cases of narrow and wide rings. The results are qualitatively compared with experimental data existing on microrings and on antidots. A smaller ring containing five electrons is also analyzed to allow for a closer comparison with a recent experiment on a two-electron quantum ring.

Surface structure and stability of PdZn and PtZn alloys: Density-functional slab model studies

Geometric parameters of binary (1:1) PdZn and PtZn alloys with CuAu-L10 structure were calculated with a density functional method. Based on the total energies, the alloys are predicted to feature equal formation energies. Calculated surface energies of PdZn and PtZn alloys show that (111) and (100) surfaces exposing stoichiometric layers are more stable than (001) and (110) surfaces comprising alternating Pd (Pt) and Zn layers. The surface energy values of alloys lie between the surface energies of the individual components, but they differ from their composition weighted averages. Compared with the pure metals, the valence d-band widths and the Pd or Pt partial densities of states at the Fermi level are dramatically reduced in PdZn and PtZn alloys. The local valence d-band density of states of Pd and Pt in the alloys resemble that of metallic Cu, suggesting that a similar catalytic performance of these systems can be related to this similarity in the local electronic structures.

Relations among several nuclear and electronic density functional reactivity indexes

A set of connections among several nuclear and electronic indexes of reactivity in the framework of the conceptual Density Functional Theory by using an expansion ofthe energy functional in terms of the total number of electrons and the normal coordinates within a canonical ensemble was derived. The relations obtained provided explicit links between important quantities related to the chemical reactivity of a system. This paper particularly demonstrates that the derivative of the electronic energy with respect to the external potential of a system in its equilibrium geometry was equal to the negative of the nuclear repulsion derivative with respect to the external potential