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  • P. Leicht, L. Zielke, S. Bouvron, R. Moroni, E. Voloshina, L. Hammerschmidt, Y. S. Dedkov, M. Fonin: In Situ Fabrication of Quasi-Free-Standing Epitaxial Graphene Nanoflakes on Gold, ACS Nano. 8, 2725-2742 (2014).DOI: 10.1021.nn500396c

Addressing the multitude of electronic phenomena theoretically predicted for confined graphene structures requires appropriate in situ fabrication procedures yielding graphene nanoflakes (GNFs) with well-defined geometries and accessible electronic properties. Here, we present a simple strategy to fabricate quasi-free-standing GNFs of variable sizes, performing temperature programmed growth of graphene flakes on the Ir(111) surface and subsequent intercalation of gold. Using scanning tunneling microscopy (STM), we show that epitaxial GNFs on a perfectly ordered Au(111) surface are formed while maintaining an unreconstructed, singly hydrogen-terminated edge structure, as confirmed by the accompanying density functional theory (DFT) calculations. Using tip-induced lateral displacement of GNFs, we demonstrate that GNFs on Au(111) are to a large extent decoupled from the Au(111) substrate. The direct accessibility of the electronic states of a single GNF is demonstrated upon analysis of the quasiparticle interference patterns obtained by low-temperature STM. These findings open up an interesting playground for diverse investigations of graphene nanostructures with possible implications for device fabrication.

  • D. Andrae: Total State Designation for Electronic States of Periodic Systems, Proceedings – 38th Annual Condensed Matter and Materials Meeting – Waiheke Island, Auckland, NZ, (2014).ISBN: 978-0-646-93339-9

The role of a complete set of commuting operators (CSCO) is first recalled with the discussion of the electronic states of two finite systems as illustrative examples. It is then shown that its role is very well transferable to sequences of finite systems that approach a real periodic system in the limit where the number of monomers becomes huge. In addition, the concept of the density of states (DOS) of total energy E, n(E-E0) (E0 is the energy of the electronic ground state), is introduced as a system’s characteristic

  • D. Tietze, St. Tietze, D. Mollenhauer, G. Buntkowsky: NMR Crystallography as a Novel Tool for the Understanding of the Mode of Action of Enzymes: SOD a Case Study, Applied Magn. Res. 45, 841 (2014).DOI: 10.1007/s00723-014-0576-9

Nuclear magnetic resonance (NMR) crystallography is an approach for revealing molecular and supramolecular structures and molecular packing for systems where standard X-ray crystallography gives no results. It combines solid-state NMR techniques with chemical models and/or molecular dynamics and/or quantum chemical calculations. These techniques are often supported by other structure characterization methods. In the present review, recent results on the application of NMR crystallography for the investigation of the mode of action of superoxide dismutases are discussed. Studies of substrate–inhibitor complexes of human manganese and Streptomyces nickel superoxide dismutase are presented, which are chemical models of the transient enzyme–substrate complex. The review is completed by new, previously unpublished results, calculating an NMR structure of NiSOD model peptide-bound cyanide based on experimental restraints measured by us and derived from the literature and extended DFT calculations.

  • D. G. Fleming, J. Manz, K. Sato, T. Takayanagi: Fundamental Change in the Nature of Chemical Bonding by Isotopic Substitution, Angew. Chem. Int. Ed. 53, 13706 (2014).DOI: 10.1002/anie.201408211

Isotope effects are important in the making and breaking of chemical bonds in chemical reactivity. Here we report on a new discovery, that isotopic substitution can fundamentally alter the nature of chemical bonding. This is established by systematic, rigorous quantum chemistry calculations of the isotopomers BrLBr, where L is an isotope of hydrogen. All the heavier isotopomers of BrHBr, BrDBr, BrTBr, and Br4HBr, the latter indicating the muonic He atom, the heaviest isotope of H, can only be stabilized as van der Waals bound states. In contrast, the lightest isotopomer, BrMuBr, with Mu the muonium atom, alone exhibits vibrational bonding, in accord with its possible observation in a recent experiment on the Mu+Br2 reaction. Accordingly, BrMuBr is stabilized at the saddle point of the potential energy surface due to a net decrease in vibrational zero point energy that overcompensates the increase in potential energy.

  • E. Fertitta, B. Paulus, G. Barcza, Ö. Legeza: Investigation of metal–insulator-like transition through the ab initio density matrix renormalization group approach, Phys. Rev. B 90, 245129 (2014).DOI: 10.1103/PhysRevB.90.245129

We have studied the metal–insulator-like transition in pseudo-one-dimensional systems, i.e., lithium and beryllium rings, through the ab initio density matrix renormalization group (DMRG) method. Performing accurate calculations for different interatomic distances and using quantum information theory, we investigated the changes occurring in the wave function between a metallic-like state and an insulating state built from free atoms. We also discuss entanglement and relevant excitations among the molecular orbitals in the Li and Be rings and show that the transition bond length can be detected using orbital entropy functions. Also, the effect of different orbital bases on the effectiveness of the DMRG procedure is analyzed comparing the convergence behavior.

  • M. Berg, A. Accardi, B. Paulus, B. Schmidt: Rotationally adiabatic pair interactions of para- and ortho-hydrogen with the halogen molecules F2, Cl2, and Br2, J. Chem. Phys. 141, 074303 (2014).DOI: 10.1063/1.4892599

The present work is concerned with the weak interactions between hydrogen and halogen molecules, i.e., the interactions of pairs H2–X2 with X = F, Cl, Br, which are dominated by dispersion and quadrupole-quadrupole forces. The global minimum of the four-dimensional (4D) coupled cluster with singles and doubles and perturbative triples (CCSD(T)) pair potentials is always a T shaped structure where H2 acts as the hat of the T, with well depths (D e ) of 1.3, 2.4, and 3.1 kJ/mol for F2, Cl2, and Br2, respectively. MP2/AVQZ results, in reasonable agreement with CCSD(T) results extrapolated to the basis set limit, are used for detailed scans of the potentials. Due to the large difference in the rotational constants of the monomers, in the adiabatic approximation, one can solve the rotational Schrödinger equation for H2 in the potential of the X2 molecule. This yields effective two-dimensional rotationally adiabatic potential energy surfaces where pH2 and oH2 are point-like particles. These potentials for the H2–X2 complexes have global and local minima for effective linear and T-shaped complexes, respectively, which are separated by 0.4-1.0 kJ/mol, where oH2 binds stronger than pH2 to X2, due to higher alignment to minima structures of the 4D-pair potential. Further, we provide fits of an analytical function to the rotationally adiabatic potentials.

  • K. G. Steenbergen, N. Gaston, C. Müller, B. Paulus: Method of increments for the halogen molecular crystals: Cl, Br, and I, J. Chem. Phys. 141, 124707 (2014).DOI: 10.1063/1.4896230

Method of increments (MI) calculations reveal the n-body correlation contributions to binding in solid chlorine, bromine, and iodine. Secondary binding contributions as well as d-correlation energies are estimated and compared between each solid halogen. We illustrate that binding is entirely determined by two-body correlation effects, which account for >80% of the total correlation energy. One-body, three-body, and exchange contributions are repulsive. Using density-fitting (DF) local coupled-cluster singles, doubles, and perturbative triples for incremental calculations, we obtain excellent agreement with the experimental cohesive energies. MI results from DF local second-order Møller-Plesset perturbation (LMP2) yield considerably over-bound cohesive energies. Comparative calculations with density functional theory and periodic LMP2 method are also shown to be less accurate for the solid halogens.

  • J. Manz, J. F. Perez-Torres, Y. Yang: Vibrating H2+(2Σg+, JM = 00) Ion as a Pulsating Quantum Bubble in the Laboratory Frame, J. Phys. Chem. A 118, 8411-8425 (2014).DOI: 10.1021/jp5017246

We present quantum dynamics simulations of the concerted nuclear and electronic densities and flux densities of the vibrating H2+ ion with quantum numbers 2Σg+, JM = 00 corresponding to the electronic and rotational ground state, in the laboratory frame. The underlying theory is derived using the nonrelativistic and Born–Oppenheimer approximations. It is well-known that the nuclear density of the nonrotating ion (JM = 00) is isotropic. We show that the electronic density is isotropic as well, confirming intuition. As a consequence, the nuclear and electronic flux densities have radial symmetry. They are related to the corresponding densities by radial continuity equations with proper boundary conditions. The time evolutions of all four observables, i.e., the nuclear and electronic densities and flux densities, are illustrated by means of characteristic snapshots. As an example, we consider the scenario with initial condition corresponding to preparation of H2+ by near-resonant weak field one-photon-photoionization of the H2 molecule in its ground state, 2Σg+, vJM = 000. Accordingly, the vibrating, nonrotating H2+ ion appears as pulsating quantum bubble in the laboratory frame, quite different from traditional considerations of vibrating H2+ in the molecular frame, or of the familiar alternative scenario of aligned vibrating H2+ in the laboratory frame.

We consider coherent tunneling of one-dimensional model systems in non-cyclic or cyclic symmetric double well potentials. Generic potentials are constructed which allow for analytical estimates of the quantum dynamics in the non-relativistic deep tunneling regime, in terms of the tunneling distance, barrier height and mass (or moment of inertia). For cyclic systems, the results may be scaled to agree well with periodic potentials for which semi-analytical results in terms of Mathieu functions exist. Starting from a wavepacket which is initially localized in one of the potential wells, the subsequent periodic tunneling is associated with tunneling velocities. These velocities (or angular velocities) are evaluated as the ratio of the flux densities versus the probability densities. The maximum velocities are found under the top of the barrier where they scale as the square root of the ratio of barrier height and mass (or moment of inertia), independent of the tunneling distance. They are applied exemplarily to several prototypical molecular models of non-cyclic and cyclic tunneling, including ammonia inversion, Cope rearrangement of semibullvalene, torsions of molecular fragments, and rotational tunneling in strong laser fields. Typical maximum velocities and angular velocities are in the order of a few km/s and from 10 to 100 THz for our non-cyclic and cyclic systems, respectively, much faster than time-averaged velocities. Even for the more extreme case of an electron tunneling through a barrier of height of one Hartree, the velocity is only about one percent of the speed of light. Estimates of the corresponding time scales for passing through the narrow domain just below the potential barrier are in the domain from 2 to 40 fs, much shorter than the tunneling times.

  • K. Steenbergen, N. Gaston: Two worlds collide: Image analysis methods for quantifying structural variation in cluster molecular dynamics, J. Chem. Phys. 140, 064102 (2014).DOI: 10.1063/1.4864753

Inspired by methods of remote sensing image analysis, we analyze structural variation in cluster molecular dynamics (MD) simulations through a unique application of the principal component analysis (PCA) and Pearson Correlation Coefficient (PCC). The PCA analysis characterizes the geometric shape of the cluster structure at each time step, yielding a detailed and quantitative measure of structural stability and variation at finite temperature. Our PCC analysis captures bond structure variation in MD, which can be used to both supplement the PCA analysis as well as compare bond patterns between different cluster sizes. Relying only on atomic position data, without requirement for a priori structural input, PCA and PCC can be used to analyze both classical and ab initio MD simulations for any cluster composition or electronic configuration. Taken together, these statistical tools represent powerful new techniques for quantitative structural characterization and isomer identification in cluster MD.

  • S. Torabi, L. Hammerschmidt, E. N. Voloshina, B. Paulus: Ab initio investigation of ground-state properties of group-12 fluorides, Int. J. Quantum Chem. (2014).DOI: 10.1002/qua.24695

The performance of wavefunction-based correlation methods in theoretical solid-state chemistry depends on reliable Hartree–Fock (HF) results for infinitly extended systems. Therefore, we optimized basis sets of valence-triple-ζ quality based on HF calculations for the periodic system of group-12-metal difluorides. Scalar-relativistic effects were included in the case of the metal-ions by applying small-core pseudopotentials. To assess the quality of the proposed basis sets, the structural parameters, bulk moduli as well as cohesive and lattice energies of the systems were evaluated at the HF and the density functional theory levels. In addition to these two mean-field approaches and to assess further employment of our basis sets to wavefunction-based correlation methods we performed periodic local MP2 computations. Finally, the possibilities of pressure induced structural phase transitions occurring in the ZnF2, CdF2, and HgF2 were investigated.

  • E. N. Voloshina, B. Paulus: First Multireference Correlation Treatment of Bulk Metals, J. Chem. Theory Comput. 10, 1698-1706 (2014).DOI: 10.1021/ct401040t

Existence of the sp–d hybridization of the valence band states of the fcc Ca and Sr in the vicinity of the Fermi level indicates that their electronic wave function can have a multireference (MR) character. We performed a wave-function-based correlation treatment for these materials by means of the method of increments. As opposed to the single-reference correlation treatment (here, coupled cluster), which fails to describe cohesive properties in both cases, employing the MR averaged coupled pair functional, one can achieve almost 100% of the experimental correlation energy.

  • L. E. Marsoner Steinkasserer, B. Paulus, E. N. Voloshina: Impact of the metal substrate on the electronic structure of armchair graphene nanoribbons, Chem. Phys. Lett. 597, 148-152 (2014).DOI: 10.1016/j.cplett.2014.02.038

Plane-wave DFT methods including semi-empirical dispersion correction are used to study in detail the interaction of different types of armchair graphene nanoribbons with Ag(1 1 1) and Au(1 1 1) surfaces. It is found that the two substrates show considerable differences in their interaction with the nanoribbons, resulting in notably different doping behavior for Au(1 1 1) and Ag(1 1 1). The obtained results are compared with the available experimental data.

  • Z. Huesges, C. Müller, B. Paulus, L. Maschio: Dispersion corrected DFT calculations for the adsorption of N2O on MgO, Surf. Science 627, 11-15 (2014).DOI: 10.1016/j.susc.2014.04.002

We have calculated adsorption energies for N2O on the MgO (001) surface using periodic DFT calculations with the B3LYP functional and subsequent dispersion correction. Additionally a wave function-based correlation treatment at the MP2 level was performed. Whilst the B3LYP calculation failed to find a bond state, both the dispersion corrections and the MP2 treatment result in a significantly better description. The best agreement with experiment is obtained with a dispersion correction via the D3 scheme. The calculated binding energies are very similar for adsorption with the nitrogen or the oxygen end towards the surface, whilst calculated vibrational frequencies of adsorbed N2O match the experimental values better when assuming an O-down adsorption structure.

  • G. Hermann, B. Paulus, J. F. Pérez-Torres, V. Pohl: Electronic and nuclear flux densities in the H2 molecule, Phys. Rev. A 89, 052504 (2014).DOI: 10.1103/PhysRevA.89.052504

We present a theoretical study of the electronic and nuclear flux densities of a vibrating H2 molecule after an electronic excitation by a short femtosecond laser pulse. The final state, a coherent superposition of the electronic ground state X 1Σ+g and the electronic excited state B 1Σ+u, evolves freely and permits the partition of the electronic flux density into two competing fluxes: the adiabatic and the transition flux density. The nature of the two fluxes allows us to identify two alternating dynamics of the electronic motion, occurring on the attosecond and the femtosecond time scales. In contradistinction to the adiabatic electronic flux density, the transition electronic flux density shows a dependence on the carrier-envelope phase of the laser field, encoding information of the interaction of the electrons with the electric field. Furthermore, the nuclear flux density displays multiple reversals, a quantum effect recently discovered by Manz et al. [J. Manz, J. F. Pérez-Torres, and Y. Yang, Phys. Rev. Lett. 111, 153004 (2013)], calling for investigation of the electronic flux density.

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