Robert Berger

Universität Stuttgart, Stuttgart, Baden-Württemberg, Germany

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Publications (48)201.48 Total impact

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    Timur Isaev · Robert Berger
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    ABSTRACT: We have outlined a rational approach to identify polyatomic molecules that appear to be promising candidates for direct Doppler cooling with lasers. Explicit numerical calculations for structures and Franck--Condon factors of selected representatives indicates high potential for laser-cooling of such molecules for even opening up the third spatial dimension for ultra-cold molecules generated by direct Doppler cooling with lasers.
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    ABSTRACT: A transformation of potential energy surfaces (PES) being represented by multi-mode expansions is introduced, which allows for the calculation of anharmonic vibrational spectra of any isotopologue from a single PES. This simplifies the analysis of infrared spectra due to significant CPU-time savings. An investigation of remaining deviations due to truncations and the so-called multi-level approximation is provided. The importance of vibrational-rotational couplings for small molecules is discussed in detail. In addition, an analysis is proposed, which provides information about the quality of the transformation prior to its execution. Benchmark calculations are provided for a set of small molecules.
    The Journal of Chemical Physics 05/2014; 140(18):184111. DOI:10.1063/1.4874849 · 3.12 Impact Factor
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    Sophie Nahrwold · Robert Berger · Peter Schwerdtfeger
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    ABSTRACT: Density functional theory within the two-component quasi-relativistic zeroth-order regular approximation (ZORA) is used to predict parity violation shifts in (183)W nuclear magnetic resonance shielding tensors of chiral, tetrahedrally bonded tungsten complexes of the form NWXYZ (X, Y, Z = H, F, Cl, Br or I), as well as for the heavier systems NWHAtF and NWH(117)F for comparison. The calculations reveal that sub-mHz accuracy is required to detect such tiny effects in this class of compounds, and that parity violation effects are very sensitive to the choice of ligands.
    The Journal of Chemical Physics 01/2014; 140(2):024305. DOI:10.1063/1.4852176 · 3.12 Impact Factor
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    ABSTRACT: The gas-phase ion chemistry of the hexachlorodisilane radical cation Si2Cl6+ has been reinvestigated by a combined electron ionization (EI) mass spectrometry (MS) and high-level quantum chemistry approach. The computational investigation of isomerization and fragmentation pathways of [Si2Cln]+() (n = 1-6) species reveals (a) the coexistence of structures of the type Cl3Si-SiClm+1+() and chlorobridged SiCl4-SiClm+() isomers for m = 1,2, and (b) a wealth of nearly isoenthalpic structures[Si2Cln]+() for each n with n = 2-4, which can result from degenerate rearrangements. This structural diversity appears to be a general feature of the disilicon (radical) cations with low chlorine content. The ionization energy of Si2Cl6 determined by EI-MS (10.1 ± 0.2stat ± 0.2syst eV) with argon (15.759 ± 0.001 eV) used as reference, is slightly lower than previously reported experimental values, but in good agreement with the CBS-QB3 (9.87 eV) and W1U (9.97 eV) adiabatic ionization energy. Measured appearance energies for the lowest two fragmentation channels, which afford SiCl2+ and SiCl3+, are well in line with CBS-QB3 values and with the literature value determined for SiCl3+ in threshold photoelectron-photoion coincidence experiments. Using the experimental heats of formation (ΔfH°298) for SiCl4 and Cl· as anchor points, W1U theory was employed to derive ΔfH°298 values for Si2Cln+() with n = 0-6 and for SiClm+() with m = 1-4 based on the lowest energy isomer identified in our quantum chemical investigation of reaction pathways for all species investigated. W1U heats of formations obtained for select neutrals lead to a more consistent description of kinetic aspects previously inferred from metastable ion mass spectra.
    International Journal of Mass Spectrometry 11/2013; 354-355:378-390. DOI:10.1016/j.ijms.2013.08.002 · 2.23 Impact Factor
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    ABSTRACT: Bijvoet's method, which makes use of anomalous x-ray diffraction or dispersion, is the standard means of directly determining the absolute (stereochemical) configuration of molecules, but it requires crystalline samples and often proves challenging in structures exclusively comprising light atoms. Herein, we demonstrate a mass spectrometry approach that directly images the absolute configuration of individual molecules in the gas phase by cold target recoil ion momentum spectroscopy after laser ionization-induced Coulomb explosion. This technique is applied to the prototypical chiral molecule bromochlorofluoromethane and the isotopically chiral methane derivative bromodichloromethane.
    Science 09/2013; 341(6150):1096-100. DOI:10.1126/science.1240362 · 31.48 Impact Factor
  • Fabian Hobi · Robert Berger · Jürgen Stohner
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    ABSTRACT: When it comes to the question of how signatures from parity violation in chiral molecules can be revealed, then ultra-high resolution spectroscopy techniques play a crucial role in attempts to detect such effects via line shifts. A prerequisite for this approach towards molecular parity violation is a detailed understanding of the corresponding conventional spectroscopy, including even small effects that are often neglected. Nuclear spin-rotation (NSR) interaction is one such effect that becomes appreciable only at high to ultra-high resolution and is often masked by nuclear electric quadrupole interactions (NQI) if nuclei are present with nuclear spin quantum number I larger than 1/2. Herein, NSR interactions are studied theoretically for the chiral molecules fluorooxirane and bromochlorofluoromethane (CHBrClF). For the latter molecule, NSR interactions are compared to nuclear electric quadrupole coupling contributions. The former molecule is devoid of I > 1/2 nuclei such that NSR interactions are not masked by NQI. Parity-odd contributions to NSR of 19F are computed that give rise to additional line shifts in hyperfine transitions. A detection of these nuclear spin-dependent P-odd terms could in principle give information on the nuclear anapole moment in fluorine, which arises due to parity violating interactions within the nucleus.
    Molecular Physics 08/2013; 111(14-15):2345-2362. DOI:10.1080/00268976.2013.816444 · 1.64 Impact Factor
  • 12/2012; DOI:10.1063/1.4771741
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    ABSTRACT: Insensitive towards inversion: Can residual dipolar couplings (RDCs) and other anisotropic NMR observables be used to determine absolute configuration? A critical assessment of recent approaches is provided to determine the absolute configuration from RDCs.
    Angewandte Chemie International Edition 08/2012; 51(33):8388-91. DOI:10.1002/anie.201107626 · 11.26 Impact Factor
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    ABSTRACT: Classical molecular dynamics simulations are a valuable tool to study the mechanisms that dominate the properties of ionic liquids (ILs) on the atomistic and molecular level. However, the basis for any molecular dynamics simulation is an accurate force field describing the effective interactions between all atoms in the IL. Normally this is done by empirical potentials which can be partially derived from quantum mechanical calculations on simple subunits or have been fitted to experimental data. Unfortunately, the number of accurate classical non-polarizable models for ILs that allow a reasonable description of both dynamical and statical properties is still low. However, the strongly increasing computational power allows one to apply computationally more expensive methods, and even polarizable-force-field-based models on time and length scales long enough to ensure a proper sampling of the phase space. This review attempts to summarize recent achievements and methods in the development of classical force fields for ionic liquids. As this class of salts covers a large number of compounds, we focus our review on imidazolium-based ionic liquids, but show that the main conclusions are valid for non-imidazolium salts, too. Insight obtained from recent electronic density functional results into the parametrization of partial charges and on the influence of polarization effects in bulk ILs is highlighted. An overview is given of different available force fields, ranging from the atomistic to the coarse-grained level, covering implicit as well as explicit modeling of polarization. We show that the recently popular usage of the ion charge as fit parameter can looked upon as treating polarization effects in a mean-field matter.
    ChemPhysChem 05/2012; 13(7):1625-37. DOI:10.1002/cphc.201100997 · 3.36 Impact Factor
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    ABSTRACT: For theoreticians, ionic liquids represent a major challenge. This is due to the fact that intermolecular interactions are particularly strong because of ionic liquids' ionicity. This, in turn, causes a subtle interplay between different scales which is encoded in the measured macro- and mesoscopic properties and also in the molecular electrostatic characteristics. Therefore, force fields have to describe the microscopic processes correctly in order to reproduce macroscopic properties accurately over a large range of state variables. Herein, imidazolium-based ionic liquids were studied at different scales, going from the detailed quantum electronic scale to the classical atomistic scale. It is indicated how the information gained at each level could be used for the other scales. In particular, the issue of deriving suitable partial charges for use in classical force fields is addressed. The Blöchl method was employed to generate partial charges reproducing the multipole distribution accurately for bulk systems. This led naturally to absolute ionic charges of less than /l e/, i.e., charge scaling. So, the monopole structure of the herein introduced force field mimics the quantum chemical behaviour observed in the liquid phase. This led to a substantial improvement in the description of dynamical properties of immediate experimental interest, such as electric conductivity. For further insight, the electric dipole moment of the ions was taken as physical indicator of their electronic structure. The electric dipole moment was found to fluctuate strongly and to depend on polarisation. Hence, our scale-combined study offers a gateway to rational design of models, based on the relevant underlying physics rather than on mere numerical parameterisation, and thereby to (possibly) more direct physical interpretation of experimental results.
    Faraday Discussions 01/2012; 154:111-32; discussion 189-220, 465-71. DOI:10.1039/C1FD00051A · 4.61 Impact Factor
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    ABSTRACT: It is shown that chirality is common in bimetallic clusters. Specifically, a detailed computational study of two copper clusters, Cun+ (n = 9,11), demonstrates that exchange of one copper atom with another metal atom (Ni, Zn, Ag, or Au) at various locations, leads, in most cases, to chirality in the a priori achiral cluster (n = 9) and always preserves it in the a priori chiral one (n = 11). Chirality was evaluated on a quantitative level, employing the Continuous Chirality Measure methodology, in two versions: a purely geometric structure analysis, and an analysis which takes into account the different nature of the atoms. Physical aspects of chirality were demonstrated by emergence of vibrational circular dichroism signals and by the emergence of parity violation (PV) energy difference, which is calculated by employing a quasi-relativistic approach. In the case of AgCu10+(p9), the PV energy splitting value is about 10–15 Hartree, bringing this nanoalloy close to the range of systems that have been discussed as promising candidates for a measurement of this phenomenon on the molecular level.
    The Journal of Physical Chemistry C 12/2011; 116(1). DOI:10.1021/jp209085r · 4.77 Impact Factor
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    Joonsuk Huh · Robert Berger
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    ABSTRACT: When existing, cumulants can provide valuable information about a given distribution and can in principle be used to either fully reconstruct or approximate the parent distribution function. A previously reported cumulant expansion approach for Franck-Condon profiles [Faraday Discuss., 150, 363 (2011)] is extended to describe also the profiles of vibronic transitions that are weakly allowed or forbidden in the Franck-Condon approximation (non-Condon profiles). In the harmonic approximation the cumulants of the vibronic spectral profile can be evaluated analytically and numerically with a coherent state-based generating function that accounts for the Duschinsky effect. As illustration, the one-photon $1 ^{1}\mathrm{A_{g}}\rightarrow1 ^{1}\mathrm{B_{2u}}$ UV absorption spectrum of benzene in the electric dipole and (linear) Herzberg-Teller approximation is presented herein for zero Kelvin and finite temperatures.
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    ABSTRACT: Three different imidazolium-based ionic liquids, 1,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium thiocyanate, and 1-ethyl-3-methylimidazolium dicyanamide, are investigated by Car–Parrinello simulations. A common behavior, such as a broad electric dipole moment distribution of the ions and a related high degree of locality, is found to characterize all these systems. Going beyond imidazolium-based systems, we found that even for the protic ionic liquid monomethyl ammonium nitrate, the same features hold. These results represent a strong support to the hypothesis of rattling ions in long-living ion cages proposed in the last years.
    Journal of Chemical Theory and Computation 09/2011; 7(10):3040--3044. DOI:10.1021/ct200375v · 5.31 Impact Factor
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    ABSTRACT: We present a detailed theoretical investigation on the dissociation energy of CuO(+), carried out by means of coupled cluster theory, the multireference averaged coupled pair functional (MR-ACPF) approach, diffusion quantum Monte Carlo (DMC), and density functional theory (DFT). At the respective extrapolated basis set limits, most post-Hartree-Fock approaches agree within a narrow error margin on a D(e) value of 26.0 kcal mol(-1) [coupled-cluster singles and doubles level augmented by perturbative triples corrections, CCSD(T)], 25.8 kcal mol(-1) (CCSDTQ via the high accuracy extrapolated ab initio thermochemistry protocol), and 25.6 kcal mol(-1) (DMC), which is encouraging in view of the disaccording data published thus far. The configuration-interaction based MR-ACPF expansion, which includes single and double excitations only, gives a slightly lower value of 24.1 kcal mol(-1), indicating that large basis sets and triple excitation patterns are necessary ingredients for a quantitative assessment. Our best estimate for D(0) at the CCSD(T) level is 25.3 kcal mol(-1), which is somewhat lower than the latest experimental value (D(0) = 31.1 ± 2.8 kcal mol(-1)[semicolon] reported by the Armentrout group) [Int. J. Mass Spectrom. 182/183, 99 (1999)]. These highly correlated methods are, however, computationally very demanding, and the results are therefore supplemented with those of more affordable DFT calculations. If used in combination with moderately-sized basis sets, the M05 and M06 hybrid functionals turn out to be promising candidates for studies on much larger systems containing a [CuO](+) core.
    The Journal of Chemical Physics 02/2011; 134(6):064304. DOI:10.1063/1.3537797 · 3.12 Impact Factor
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    Joonsuk Huh · Robert Berger
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    ABSTRACT: In this contribution, advantages and disadvantages of the time-independent and time-dependent approaches for Franck-Condon profile calculations are discussed within the displaced-distorted-rotated harmonic oscillator approximation. Particular strengths and prospects of a previously developed time-independent cumulant expansion in the calculation of a Franck-Condon profile for UV/Vis absorption spectra are demonstrated for the specific case of the S0(1Ag) --> S1(1B3u) transition of terrylene at various temperatures.
    Faraday Discussions 01/2011; 150:363-73; discussion 391-418. DOI:10.1039/C0FD00014K · 4.61 Impact Factor
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    ABSTRACT: We present a detailed calculation of partial charges for the 1,3-dimethylimidazolium chloride ionic liquid. We first analyze MP2 electronic structure calculations and DFT results on isolated ion pairs with various methods of assigning partial charges to the atomic centers. In a second run we analyze the trajectory of a 25 ps long Car-Parrinello MD run of 30 ion pairs under bulk conditions using a charge fitting procedure due to Blöchl. Both, the single ion pair and the bulk system, provide us with a similar total ionic charge considerably less than unity. Especially the liquid state DFT results give convincing evidence for a reduced ionic charge on the ions. The similarity of both results suggest that the delocalization of the Cl charge is due only to local interactions. The relevance of our results is 2-fold; on the one hand they shed light on the basic property of the liquid and its reduced ionic character, and on the other hand, the ab initio derived partial charges provide a fundamental theoretical basis for the recent attempts to use the total ionic charge as an adjustable parameter. Furthermore, all our partial charges are subject to large fluctuations, hinting to the importance of polarization effects.
    The Journal of Physical Chemistry B 05/2010; 114(18):6150-5. DOI:10.1021/jp910771q · 3.30 Impact Factor
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    ABSTRACT: We present a Lagrangian approach for the calculation of molecular (quadratic) response properties that can be expressed as geometric gradients of a generic linear response function, its poles, and its residues. The approach is implemented within an atomic-orbital-based formalism suitable for linear scaling at the level of self-consistent time-dependent Hartree−Fock and density functional theory. Among the properties that can be obtained using this formalism are the gradient of the frequency-dependent polarizability (e.g., Raman intensities) and that of the one-photon transition dipole moment (entering the Herzberg−Teller factors), in addition to the excited-state molecular forces required for excited-state geometry optimizations. Geometric derivatives of ground-state first-order properties (e.g., IR intensities) and excited-state first-order property expressions are also reported as byproducts of our implementation. The one-photon transition moment gradient is the first analytic implementation of the one-photon transition moment derivative at the DFT level of theory. Besides offering a simple solution to overcome phase (hence, sign) uncertainties connected to the determination of the Herzberg−Teller corrections by numerical derivatives techniques based on independent calculations, our approach also opens the possibility to determine, for example by a mixed analytic−numerical approach, the one-photon transition dipole Hessian, and thus to investigate vibronic effects beyond the linear Herzberg−Teller approximation. As an illustrative application, we report a DFT study of the vibronic fine structure of the one-photon X̃(1A1g) − Ã(1B2u) transition in the absorption spectrum of benzene, which is Franck−Condon-forbidden in the electric dipole approximation and hence determined by the Herzberg−Teller integrals and electronic transition dipole-moment derivatives.
    Journal of Chemical Theory and Computation 03/2010; 6(4). DOI:10.1021/ct900506c · 5.31 Impact Factor
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    ABSTRACT: Fortschritte bei vibronischen Strukturmethoden ermöglichen Einblicke in Schwingungs- und Elektronenübergnge von Systemen mit Hunderten von Freiheitsgraden. Durch Kombination von klassischen und quantenmechanischen Zugngen in einem Multiskalenansatz lassen sich quantendynamische Prozesse in komplexen Systemen auf atomarer Skala simulieren. Mit linear-skalierenden Methoden sind selbst auf Workstation-Computern Moleküle mit mehr als 1000 Atomen auf HF-, DFT- und MP2-Niveau berechenbar.
    Nachrichten aus der Chemie 03/2010; 58(3):331 - 338. DOI:10.1002/nadc.201068406 · 0.20 Impact Factor
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    ABSTRACT: In this paper a multiscale approach for the description of ionic liquids and first results according to this scheme are presented. The basic idea is to combine post-Hartree–Fock calculations (pHF), density functional theory (DFT) and classical all-atomistic molecular dynamics simulations to go from individual molecules to bulk properties in a setup that is able to bridge the large time and length scale gaps involved. The idea is initially applied to a very simple, but well studied ionic liquid compound, namely dimethylimidazolium chloride. Results obtained on the post-Hartree–Fock level for one ion pair are compared to Car–Parrinello density functional results. Subsequently density functional results for several ion pairs up to 30 ion pairs are determined, that are intended to approach bulk features. The DFT results are finally compared to molecular dynamics simulations. A crucial step in this multiscale scheme is the analysis of the electrostatic potentials. The Blöchl approach employed herein on the DFT level allows us to obtain values for partial charges, that are considerably different from the ones used in the standard force fields. The results suggest that the total charge of the cation and the anion should be scaled down to a value of around ±0.6e to ±0.8e. The usage of these partial charges within a standard force field yields structural and dynamical results that approach the structural results of the density functional calculations. Although this appears promising, another refinement step of the force field parameters will be necessary, as the density of mass obtained deviates by 15% from the experimental results. Possible further refinements of the force fields are discussed.
    Journal of Molecular Liquids 03/2010; 152:2. DOI:10.1016/j.molliq.2009.06.014 · 2.52 Impact Factor
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    Joonsuk Huh · Michael Neff · Guntram Rauhut · Robert Berger
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    ABSTRACT: Explicitly electron correlating coupled cluster calculations, CCSD(T)-F12a, were performed to determine three-dimensional potential energy hypersurfaces of disulphanide and disulphanyl in an automated approach. Surfaces for different electronic states were used in a Watson rovibrational Hamiltonian ansatz to obtain the correlated anharmonic vibrational wavefunctions. Subsequently the anharmonic Franck-Condon overlap integrals were evaluated. The computed Franck-Condon profiles were compared to experimental photodetachment-photoelectron spectra and confirm essentially the assignments made previously. The profiles indicate, however, additional weaker, and as of yet unresolved, additional features.
    Molecular Physics 02/2010; 108(3-4). DOI:10.1080/00268970903521178 · 1.64 Impact Factor

Publication Stats

722 Citations
201.48 Total Impact Points

Institutions

  • 2014
    • Universität Stuttgart
      • Institute of Theoretical Chemistry
      Stuttgart, Baden-Württemberg, Germany
  • 2010–2014
    • Technical University Darmstadt
      • Clemens Schöpf Institut für Organische Chemie und Biochemie
      Darmstadt, Hesse, Germany
  • 2006–2012
    • Goethe-Universität Frankfurt am Main
      • Frankfurt Institute for Advanced Studies
      Frankfurt, Hesse, Germany
    • University of Leipzig
      • Institute of Analytical Chemistry
      Leipzig, Saxony, Germany
  • 2003–2005
    • Technische Universität Berlin
      • Department of Chemistry
      Berlin, Land Berlin, Germany