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Addressing the multitude of electronic phenomena theoretically predicted for confined graphene structures requires appropriate in situ fabrication procedures yielding graphene nanoflakes (GNFs) with welldefined geometries and accessible electronic properties. Here, we present a simple strategy to fabricate quasifreestanding 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 hydrogenterminated edge structure, as confirmed by the accompanying density functional theory (DFT) calculations. Using tipinduced 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 lowtemperature STM. These findings open up an interesting playground for diverse investigations of graphene nanostructures with possible implications for device fabrication.
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(EE_{0}) (E_{0} is the energy of the electronic ground state), is introduced as a system’s characteristic
Nuclear magnetic resonance (NMR) crystallography is an approach for revealing molecular and supramolecular structures and molecular packing for systems where standard Xray crystallography gives no results. It combines solidstate 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 peptidebound cyanide based on experimental restraints measured by us and derived from the literature and extended DFT calculations.
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 Br^{4}HBr, 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+Br_{2} 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.
We have studied the metal–insulatorlike transition in pseudoonedimensional 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 metalliclike 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.
The present work is concerned with the weak interactions between hydrogen and halogen molecules, i.e., the interactions of pairs H_{2}–X_{2} with X = F, Cl, Br, which are dominated by dispersion and quadrupolequadrupole forces. The global minimum of the fourdimensional (4D) coupled cluster with singles and doubles and perturbative triples (CCSD(T)) pair potentials is always a T shaped structure where H_{2} acts as the hat of the T, with well depths (D e ) of 1.3, 2.4, and 3.1 kJ/mol for F_{2}, Cl_{2}, and Br_{2}, 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 H_{2} in the potential of the X_{2} molecule. This yields effective twodimensional rotationally adiabatic potential energy surfaces where pH_{2} and oH_{2} are pointlike particles. These potentials for the H_{2}–X_{2} complexes have global and local minima for effective linear and Tshaped complexes, respectively, which are separated by 0.41.0 kJ/mol, where oH2 binds stronger than pH_{2} to X_{2}, due to higher alignment to minima structures of the 4Dpair potential. Further, we provide fits of an analytical function to the rotationally adiabatic potentials.
Method of increments (MI) calculations reveal the nbody correlation contributions to binding in solid chlorine, bromine, and iodine. Secondary binding contributions as well as dcorrelation energies are estimated and compared between each solid halogen. We illustrate that binding is entirely determined by twobody correlation effects, which account for >80% of the total correlation energy. Onebody, threebody, and exchange contributions are repulsive. Using densityfitting (DF) local coupledcluster singles, doubles, and perturbative triples for incremental calculations, we obtain excellent agreement with the experimental cohesive energies. MI results from DF local secondorder MøllerPlesset perturbation (LMP2) yield considerably overbound cohesive energies. Comparative calculations with density functional theory and periodic LMP2 method are also shown to be less accurate for the solid halogens.
We present quantum dynamics simulations of the concerted nuclear and electronic densities and flux densities of the vibrating H_{2}^{+} ion with quantum numbers ^{2}Σ_{g}^{+}, 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 wellknown 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 nearresonant weak field onephotonphotoionization of the H_{2} molecule in its ground state, ^{2}Σ_{g}^{+}, vJM = 000. Accordingly, the vibrating, nonrotating H_{2}^{+} 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 H_{2}^{+} in the laboratory frame.
We consider coherent tunneling of onedimensional model systems in noncyclic or cyclic symmetric double well potentials. Generic potentials are constructed which allow for analytical estimates of the quantum dynamics in the nonrelativistic 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 semianalytical 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 noncyclic 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 noncyclic and cyclic systems, respectively, much faster than timeaveraged 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.
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.
The performance of wavefunctionbased correlation methods in theoretical solidstate chemistry depends on reliable Hartree–Fock (HF) results for infinitly extended systems. Therefore, we optimized basis sets of valencetripleζ quality based on HF calculations for the periodic system of group12metal difluorides. Scalarrelativistic effects were included in the case of the metalions by applying smallcore 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 meanfield approaches and to assess further employment of our basis sets to wavefunctionbased correlation methods we performed periodic local MP2 computations. Finally, the possibilities of pressure induced structural phase transitions occurring in the ZnF_{2}, CdF_{2}, and HgF_{2} were investigated.
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 wavefunctionbased correlation treatment for these materials by means of the method of increments. As opposed to the singlereference 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.
Planewave DFT methods including semiempirical 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.
We have calculated adsorption energies for N_{2}O on the MgO (001) surface using periodic DFT calculations with the B3LYP functional and subsequent dispersion correction. Additionally a wave functionbased 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 N_{2}O match the experimental values better when assuming an Odown adsorption structure.
We present a theoretical study of the electronic and nuclear flux densities of a vibrating H_{2} molecule after an electronic excitation by a short femtosecond laser pulse. The final state, a coherent superposition of the electronic ground state X ^{1}Σ^{+}_{g} and the electronic excited state B ^{1}Σ^{+}_{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 carrierenvelope 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. PérezTorres, and Y. Yang, Phys. Rev. Lett. 111, 153004 (2013)], calling for investigation of the electronic flux density. 
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This page was last modified on February 05, 2016, at 11:15 AM 