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  • H.-Ch. Hege, J. Manz, F. Marquardt, B. Paulus, A. Schild, "Electron flux during pericyclic reactions in the tunneling limit: Quantum simulation for cyclooctatetraene", Chem. Phys. 376, 46 (2010).

Pericyclic rearrangement of cyclooctatetraene proceeds from equivalent sets of two reactants to two products. In the ideal limit of coherent tunneling, these reactants and products may tunnel to each other by ring inversions and by double bond shifting (DBS). We derive simple cosinusoidal or sinusoidal time evolutions of the bond-to-bond electron fluxes and yields during DBS, for the tunneling scenario. These overall yields and fluxes may be decomposed into various contributions for electrons in so called pericyclic, other valence, and core orbitals. Pericyclic orbitals are defined as the subset of valence orbitals which describe the changes of Lewis structures during the pericyclic reaction. The quantum dynamical results are compared with the traditional scheme of fluxes of electrons in pericyclic orbitals, as provided by arrows in Lewis structures. DOI: 10.1016/j.chemphys.2010.07.033

  • K. Rosciszewski; B. Paulus, "The zero-point energy in the molecular hydrogen crystal", Molecular Phys. 108, 2147 (2010).

We propose a modified Einstein approximation to describe zero-point energy vibrations in a quantum crystal. Our aim was to develop a computationally cheap tool suitable for lattice structure optimisation. As in the classical Einstein model the representative atom vibrates in an effective potential due to the surrounding atoms of the crystal; the atoms however are not strictly placed at the positions corresponding to the crystal potential energy minima but their positions are described by the quantum mechanical density distributions. The effective potential computed that way is suitable for the application in solid para-hydrogen in contrast to the normal (unmodified) Einstein approximation. We compute the cohesive energy of the para-hydrogen crystal and perform lattice structure optimisation. The hexagonal closed packed is more stable than the fcc closed packed lattice and the lattice constants obtained are in very good agreement with the experimental values. DOI:

  • S. Mebs, S. Grabowsky, D. Frster, R. Kickbusch, M. Hartl, L. L. Daemen, W. Morgenroth, P. Luger, B. Paulus, and D. Lentz, "Charge transfer via the dative N-B bond and dihydrogen contacts. Experimental and theoretical electron density studies of small Lewis acid-base adducts", J. Phys. Chem. A 114, 10185 (2010). DOI: 10.1021/jp100995n

The electronic characteristics of the dative N-B bond in three Lewis acid-base adducts, hydrazine borane, hydrazine bisborane, and ammonia trifluoroborane, are analyzed by an approach combining experimental electron density determination with a broad variety of theoretical calculations. Special focus is directed to the weak dihydrogen contacts in hydrazine borane. The Atoms In Molecules partitioning scheme is complemented by additional methods like the Source Function, and the Electron Localizability Indicator. For the multipole-free theoretical models of hydrazine borane and hydrazine bisborane, a weak charge donation from Lewis base to acid of about 0.05 e is found, whereas multipole refinement of theoretical and experimental structure factors resulted in opposite signs for the Lewis acid and base fragments. For ammonia trifluoroborane, the donation from Lewis base to acid is slightly larger (about 0.13 e) in the multipole-free models, and the charges obtained by multipole refinement retain the direction of the charge donation but show quite large variations. The natural population analysis charges predict larger charge donations (0.35 e) from the Lewis bases to the acids for the three title complexes. Although the three compounds exhibit intermolecular interactions of different types and strengths, including classical hydrogen bonds, F···H contacts and the already mentioned dihydrogen bonds, almost no charge transfer is detected between different molecules within the crystal environment. The main electronic effect of the formation of the Lewis acid−base adducts and of the crystallization is an increase in the charge separation within the ammonia/hydrazine fragments, which is supported by all investigated bond and atomic properties. The nature of the dative N-B bond is found to be mainly electrostatic, but with a substantial contribution of covalency. The F−B bonds show similarities and differences from the N-B bonds, which makes a distinction of coordinative (or dative) bonds from polar covalent interactions possible.

  • B. Paulus and H. Stoll, "The method of increments - a wavefunction-based correlation method for extended systems", in Accurate Condensed-Phase Quantum Chemistry, ed. F Manby (CRC Press, 2010). ISBN 978-1-4398-0837-5
  • E. Voloshina and B. Paulus, "Development of a wavefunction-based ab initio method for metals applying the method of increments", in Progress in Physical Chemistry, Vol. 3, ed. M. Dolg (Oldenbourg Wissenschaftsverlag GmbH, Muenchen, 2010). ISBN 978-3-486-59827-8

Ab initio electron correlation calculations based on quantum-chemical methods are successfully applied to metallic systems via the method of increments, which is an expansion of the correla- tion energy in terms of one-body, two-body, and higher-order contributions from localized orbital groups. To deal with the two distinct problems that occur in metals, the difficulty of localization of the orbitals and the generation of clusters with neutral atoms in the center, we proposed an embedding scheme which has itself no metallic character but can mimic the metal in the internal region, where the atoms are correlated. The first application was made for solid mercury, where a very good agreement with experimental ground-state properties was achieved. Further the approach has been extended to other group 2 and 12 metals (Be, Mg, Zn, and Cd) where the metallic character is more pronounced than in mercury. Different variants of the embedding and the localized orbitals used for the incremental scheme have been developed and tested. Application of the method of increments to the investigated metallic systems allows us not only to obtain values close to experimental data but also to understand the influence of individual correlation-energy increments on cohesive properties and to clarify thereby some aspects of the structural features of the group 12 metals.

  • D. Belger, Z. Hüsges, E. Voloshina, and B. Paulus, The role of electron correlations in the binding properties of Ca, Sr, and Ba, J. Phys.: Condens. Matter 22, 275504 (2010).DOI: 10.1088/0953-8984/22/27/275504

In order to apply wavefunction-based correlation methods to solids it is necessary to have reliable Hartree–Fock (HF) results for the infinite system of interest. We performed Hartree–Fock calculations for the group 2 heavy alkali-earth metals Ca, Sr, and Ba. For that, basis sets of valence-double-ζ quality have been optimized for the periodic systems. In all cases small-core pseudopotentials were used to deal with the scalar-relativistic effects. We determine the cohesive energies, the equilibrium volumes and the bulk moduli of the systems at the Hartree–Fock level and compare them with experimental data as well as the results of density functional theory calculations. Relativistic effects in the case of Ba are estimated by using a non-relativistic pseudopotential. The comparative HF versus the density functional theory (DFT) study of the electronic structures of Ca, Sr, and Ba has been performed.

  • E. Voloshina and B. Paulus, Development of a wavefunction-based ab initio method for metals applying the method of increments, Z. Phys. Chem. 224 369 (2010). DOI: 10.1524.zpch.2010.6111

Ab initio electron correlation calculations based on quantum-chemical methods are successfully applied to metallic systems via the method of increments, which is an expansion of the correla- tion energy in terms of one-body, two-body, and higher-order contributions from localized orbital groups. To deal with the two distinct problems that occur in metals, the difficulty of localization of the orbitals and the generation of clusters with neutral atoms in the center, we proposed an embedding scheme which has itself no metallic character but can mimic the metal in the internal region, where the atoms are correlated. The first application was made for solid mercury, where a very good agreement with experimental ground-state properties was achieved. Further the approach has been extended to other group 2 and 12 metals (Be, Mg, Zn, and Cd) where the metallic character is more pronounced than in mercury. Different variants of the embedding and the localized orbitals used for the incremental scheme have been developed and tested. Application of the method of increments to the investigated metallic systems allows us not only to obtain values close to experimental data but also to understand the influence of individual correlation-energy increments on cohesive properties and to clarify thereby some aspects of the structural features of the group 12 metals.

  • A. Kenfack, F. Marquardt, G. K. Paramonov, I. Barth, C. Lasser, and B. Paulus, Initial-state dependence of coupled electronic and nuclear fluxes in molecules, Phys. Rev. A, 81, 052502 (2010). DOI: 10.1103/PhysRevA.81.052502

We demonstrate that coupled electronic and nuclear fluxes in molecules can strongly depend on the initial state preparation. Starting the dynamics of an aligned D'_2-'+ molecule at two different initial conditions, the inner and the outer turning points, we observe qualitatively different oscillation patterns of the nuclear fluxes developing after 30 fs. This corresponds to different orders of magnitude bridged by the time evolution of the nuclear dispersion. Moreover, there are attosecond time intervals within which the electronic fluxes do not adapt to the nuclei motion depending on the initial state. These results are inferred from two different approaches for the numerical flux simulation, which are both in good agreement.

  • D. Mollenhauer, I. Corral, and L. González, Four plus Four State Degeneracies in the O-O Photolysis of Aromatic Endoperoxides, J. Phys. Chem. Lett., 1, 1036 (2010). DOI: 10.1021/jz100196q

The thermal and photochemical O−O bond dissociation mechanisms in the aromatic oxygen carrier photosensitizer anthracene-9,10-endoperoxide have been elucidated using high-level multiconfigurational ab initio calculations (CASSCF and MS-CASPT2). Our results show that in both the thermal and the photochemical pathways, the system proceeds through a degeneracy of four singlet plus four triplet states. This high-order degeneracy provides an efficient funnel for radiationless deactivation from the lowest excited state, indicating that the system does not dissociate in the excited-state manifold but that the products are rather formed in the electronic ground state. Accordingly, the homolysis of the peroxide group leads to four plus four ground-state biradicals, which are the precursors of the experimentally observed rearrangement products.

  • J. Kullgren, Ch. W. M. Castleton, C. Müller, D. M. Ramo, and K. Hermansson, B3LYP calculations of cerium oxides, J. Chem. Phys. 132, 054110 (2010). DOI:10.1063/1.3253795

In this paper we evaluate the performance of density functional theory with the B3LYP functional for calculations on ceria (CeO2) and cerium sesquioxide (Ce2O3). We demonstrate that B3LYP is able to describe CeO2 and Ce2O3 reasonably well. When compared to other functionals, B3LYP performs slightly better than the hybrid functional PBE0 for the electronic properties but slightly worse for the structural properties, although neither performs as well as LDA+U(U=6 eV) or PBE+U(U=5 eV). We also make an extensive comparison of atomic basis sets suitable for periodic calculations of these cerium oxides. Here we conclude that there is currently only one type of cerium basis set available in the literature that is able to give a reasonable description of the electronic structure of both CeO2 and Ce2O3. These basis sets are based on a 28 electron effective core potential (ECP) and 30 electrons are attributed to the valence space of cerium. Basis sets based on 46 electron ECPs fail for these materials.

  • Yu. S. Dedkov, E. N. Voloshina, and M. Richter, Preparation and photoemission investigation of bulklike α-Mn films on W(110), Phys. Rev. B. 81, 085404 (2010). DOI:10.1103/PhysRevB.81.085404

We report the successful stabilization of a thick bulklike distorted α-Mn film with (110) orientation on a W(110) substrate. The observed (3×3) overstructure for the Mn film with respect to the original W(110) low-energy electron-diffraction pattern is consistent with the presented structure model. The possibility to stabilize such a pseudomorphic Mn film is supported by density-functional total-energy calculations. Angle-resolved photoemission spectra of the stabilized α-Mn(110) film show weak dispersions of the valence-band electronic states in accordance with the large unit cell.
The article was selected for the February 15, 2010 issue of Virtual Journal of Nanoscale Science & Technology.

  • L. Siurakshina, B. Paulus, V. Yushankhai, and E. Sivachenko, Quantum chemical study of Co3+ spin states in LaCoO3, Eur. Phys. J. B 74, 53 (2010). DOI:10.1140/epjb/e2010-00063-0

Ab initio quantum-chemical cluster calculations are performed for the perovskite LaCoO3. The main concern is to calculate the energy level ordering of different spin states of Co3+, which is an issue of great controversy for many years. The calculations performed for the trigonal lattice structure at T = 5 K and 300 K, with the structural data taken from experiment, display that the low-spin (LS, S = 0) ground state is separated from the first excited high-spin (HS, S = 2) state by a gap <100 meV, while the intermediate-spin (IS, S = 1) state is located at much higher energy ≈0.5 eV. We suggest that the local lattice relaxation around the Co3+ ion excited to the HS state and the spin-orbit coupling reduce the spin gap to a value ~10 meV. Coupling of the IS state to the Jahn-Teller local lattice distortion is found to be rather strong and reduces its energy position to a value of 200...300 meV. Details of the quantum-chemical cluster calculation procedure and the obtained results are extensively discussed and compared with those reported earlier by other authors.

  • M. Weser, Y. Rehder, K. Horn, M. Sicot, M. Fonin, A. B. Preobrajenski, E. N. Voloshina, E. Goering, and Yu. S. Dedkov, Induced magnetism of carbon atoms at the graphene/Ni(111) interface, Appl. Phys. Lett. 96, 012504 (2010). DOI:10.1063/1.3280047

We report an element-specific investigation of electronic and magnetic properties of the graphene/Ni(111) system. Using x-ray magnetic circular dichroism, the occurrence of an induced magnetism of the carbon atoms in the graphene layer is observed. We attribute this magnetic moment to the strong hybridization between C π and Ni 3d valence band states. The net magnetic moment of carbon in the graphene layer is estimated to be in the range of 0.05 − 0.1 µB per atom.

  • N. Gaston, D. Andrae, B. Paulus, U. Wedig, and M. Jansen: Understanding the hcp anisotropy in Cd and Zn: the role of electron correlation in determining the potential energy surface, Phys. Chem. Chem. Phys. 12, 681 (2010). DOI: 10.1039/b915967c

Cadmium crystallises in the hcp structure, but with an anomalously large c/a ratio, indicating a strong distortion away from ideal packing. Coupled cluster calculations within the framework of the method of increments with an embedding scheme for metals were performed to explore the potential energy surface of cadmium with respect to the hexagonal lattice parameters. This potential energy surface is compared to density functional theory based surfaces, as calculated with various functionals. The overall flatness of the potential energy surface over a wide range of values of the lattice parameter c is analogous for both treatments, however only within the method of increments do we quantitatively describe the cohesion. The overall behaviour of the method of increments for cadmium is consistent with previous results for zinc, emphasising the dominant role of electronic correlation in achieving a sufficiently accurate description of bonding properties for the two elements; however, a detailed analysis shows differences. We discuss this in detail in terms of the correlation contributions of the s- and d-electrons.

  • A. Kenfack and K. P. Singh: From physics to control through an emergent view, World Scientific Series B, 15, 63 (2010).
  • U. E. Vincent, A. Kenfack, D. V. Senthilkumar, D. Mayer and J. Kurths: Current reversals and synchronization in coupled ratchets, Phys. Rev. E 82, 046208 (2010).

Current reversal is an intriguing phenomenon that has been central to recent experimental and theoretical investigations of transport based on ratchet mechanism. By considering a system of two interacting ratchets, we demonstrate how the coupling can be used to control the reversals. In particular, we find that current reversal that exists in a single driven ratchet system can ultimately be eliminated with the presence of a second ratchet. For specific coupling strengths a current-reversal free regime has been detected. Furthermore, in the fully synchronized state characterized by the coupling threshold kth, a specific driving amplitude aoptis found for which the transport is optimum.

  • A. Kenfack and K. P. Singh: Stochastic resonance in coupled underdamped bistable system, Phys. Rev. E 82, 046224 (2010).

We study onset and control of stochastic resonance (SR) phenomenon in two driven bistable systems, mutually coupled and subjected to independent noises, taking into account the influence of both the inertia and the coupling. In the absence of coupling, we found two critical damping parameters: one for the onset of SR and another for which SR is optimum. We then show that in weakly coupled systems, emergence of SR is governed by chaos. A strong coupling between the two oscillators induces coherence in the system; however, the systems do not synchronize no matter what the coupling is. Moreover, a specific coupling parameter is found for which the SR of each subsystem is optimum. Finally, a scheme for controlling SR in such coupled systems is proposed by introducing a phase difference between the two coherent driving forces.

  • A. Kenfack, I. Barth, F. Marquardt and B. Paulus: Molecular isotopic effects on coupled electronic and nuclear fluxes, Phys. Rev. A 82, 062502 (2010).

A full quantum treatment shows that coupled electronic and nuclear fluxes exhibit a strong sensitivity to a small mass change in a vibrating molecule. This has been exemplified with the existing isotopes of H2+ as well as few fictitious ones. We find that the fluxes undergo a significant change as one goes from one isotope of reduced mass µ to another. Other well-defined observables are likewise affected. It turns out that as a general rule, the heavier the isotope, the larger the flux, the smaller the dispersion, and the longer the revival period. While we were able to confirm analytically that the time at the first turning point scales as √µ and that the revival period changes linearly with µ, the mechanism of other observables remains subtle as the result of quantum interference highlighted by the pronounced difference observed on the dispersion pattern.

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