The brochure of the John von Neumann Institute for Computing is available in English and in German. It can be ordered at the NIC secretariat (nic@fz-juelich.de).

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Intro- duction	Scientific Computing	Astro- physics	Elementary Particles	Multi- particles	Polymers	Chemistry	Earth, En- vironment	Other Fields

    Chemistry

• "Chemistry"
• Structures and Properties of Nanostructured Clusters of Transition Metal Oxides
• Rearrangements of Molecular Complexes - The Compliance Matrices Method as a Modern Prognostic Instrument
• Switchable Dyes for Nonlinear Optics
• Photostability of DNA
• Electronic Conduction on the Molecular Scale
• Transition Metal Ions in Biological Matrices

"Chemistry"

Chemistry, that is the investigation of the properties of substances and their transformations into each other, has been based for centuries on a meticulous observation of nature and on imaginative experiments. Knowledge of the structure of matter composed of positively and negatively charged nuclei and electrons and the development of quantum mechanics permitted the formulation of a mathematical equation from which, in principle, all the observable properties of atomic and molecular systems can be determined. This paved the way for a mathematical treatment of chemical processes. However, this equation is so complicated that only a solution by approximation is possible for a many-particle system. Promoted by the rapid development of computers, this field of quantum chemistry has undergone a rapid development since the sixties. Calculations today permit reliable statements on chemical systems with a precision entirely comparable to that of experimental data. Such calculations can also provide information about systems or processes not directly accessible to experiments. Nevertheless, experiments are not superfluous since methods and theoretical predictions are improved in a constant dialogue between experiment and theory.

The methods used range from quantum chemical "ab initio" techniques, aiming at a solution of quantum chemical equations without empirical data, through density functional methods using a few parameters, up to more approximative methods of molecular mechanics or molecular dynamics using classical equations of motion and force fields obtained empirically or by quantum mechanics. Depending on system size and computer effort, various "hybrid techniques" are also applied. In particular QM/MM methods, combining the quantum mechanical (QM) description (critical for binding properties) with molecular mechanics (MM) treatments (for the part of the system only slightly perturbed by a process) have become very popular today. Similarly a combination of density functional theory (DFT) and molecular dynamics (MD) is frequently used to investigate complex reactions.

Computational chemistry is today a central tool in the study of new materials. Calculations predict structures and stabilities of as yet unknown molecules and help to design strategies for the appropriate synthesis strategies. This is particularly important in the investigation and design of nanostructured materials, since calculations can systematically vary the cluster size from small molecules to the bulk. In this context, electrical and optical properties of such materials are also of great interest, for example in the search for high-density optical data storage and molecular switches. Electronic devices are becoming increasingly microscopic and will eventually approach the single-molecule level. Modern quantum chemical treatments are in a position to compute the electric current through a single (organic) molecule and can give guidelines on how to prepare molecules with corresponding optimal circuit properties.

Chemical reactions are studied on the molecular level by computing the minimal energy path from primary to final products. Details of transition states, rearrangements and the occurrence of possible short-lived intermediates along the reaction path are generally very difficult to determine experimentally and hence quantum chemical computations are central for the understanding of many processes. The computational study of catalytic and enzymatic reactions requires large computer resources. Energy differences between various conformations of biomolecules are often very small (a few kJ/mol), and weak hydrogen bonding and the interaction of the molecule with the solvent is generally present. Nevertheless, calculations can often point out the key quantity to understand a given process, for example, why and in what manner nature utilizes one given biological matrix to perform two or more different chemical reactions.

The calculation of electronically excited states of large molecules is a special challenge. There are many examples in the literature which show the power of computational chemistry to investigate photochemical radical reactions in our atmosphere, including the influence of pollutants. Simulation of photo-induced chemical processes in DNA building blocks, an example given here, requires the most powerful theoretical procedures and computational facilities.

The awarding of the 1998 chemistry Nobel prize to Walter Kohn and John A. Pople acknowledges the path-breaking achievements of these scientists in the field of quantum chemistry, but also documents the significance of this domain of research for chemistry as a whole. Hardly any field from molecular physics through inorganic and organic chemistry up to and including biochemistry can today dispense with the application of computer methods. The provision of powerful computers is an essential prerequisite.

(Sigrid Peyerimhoff, Theoretical Chemistry, University of Bonn)

Structures and Properties of Nanostructured Clusters of Transition Metal Oxides

One of the fundamental questions in understanding materials is how the structure, properties and reactivity of a chemical compound change when passing from small molecules through nano-sized clusters (hundreds to thousands of atoms) up to bulk solids. The full characterization of cluster structures remains an experimental challenge in which the use of quantum chemical methods is indispensable. Current investigations focus on the structure and stability of gas-phase vanadium oxide clusters of different sizes and shapes. Calculations indicate that for small vanadium pentoxide clusters cage-like structures (left) are energetically favored in comparison to molecular fragments of the bulk structure (right). The question arises of the cluster size at which this energetic ordering is reversed and bulk structure as well as bulk properties are approached. Due to the problem size, the project relies on a recently developed massively parallel density functional theory based code which, in a cooperation between the University of Karlsruhe and ZAM, has been implemented in the TURBOMOLE quantum chemical program package.

(Thomas Müller, NIC-ZAM, Jülich; Marek Sierka, Jens Döbler and Joachim Sauer, Humboldt University, Berlin)

Rearrangements of Molecular Complexes - The Compliance Matrices Method as a Modern Prognostic Instrument

In synthetically important molecular rearrangement reactions the question arises of the strengths of bonds to be broken and to be newly formed. There is a need for unique descriptors of bond strengths, the numerical values of which are independent of the coordinate system used. The project aims at establishing the method of compliance matrices as a tool to routinely describe and predict bond strengths in elementorganic and metallorganic compounds.

The diagram shows the rearrangement (1,2-silatropy) of intermediate conformeric tungsten complexes (on the left, one conformer chosen) via a transition state exhibiting a penta- coordinate silicon center with weak Si-P- and Si-C-bonds. The method of compliance matrices predicts a weak P-Si-bond in the products (on the right), which is of particular interest for obtaining further insight into reactivity pathways.

(Rainer Streubel, Institute of Inorganic Chemistry, University of Bonn)

Switchable Dyes for Nonlinear Optics

Azobenzene dyes have attracted much interest in recent years due to their unusual optical and photochemical properties, which make them interesting candidates for applications in fields such as high-density optical data storage or fast telecommunication technologies. By irradiation with visible or ultraviolet light, azobenzene dyes can be switched reversibly between two distinct states with largely different structural and optical properties. The light wavelength necessary for the switching process as well as other properties can be tuned to a considerable extent by rather simple modifications in the chemical structure of the dyes. Currently, the effect of such changes on the linear and nonlinear optical properties of substituted azobenzene dyes are being investigated using quantum chemical methods based on time-dependent density functional theory.

The figure shows the highest occupied and lowest unoccupied molecular orbitals of a push- pull substituted azobenzene dye, which can be regarded as approximate visualizations of the ground and excited states of the molecule. These electronic states are involved in both the photochemical isomerization mechanism and the nonlinear susceptibility of the azobenzene dye. A detailed knowledge of the electronic states, as accessible by quantum chemical calculations, allows the prediction of new dyes with improved properties for various applications. The simulations along these lines are currently being performed on the Jump supercomputer at NIC.

(Wolfgang Hieringer, Theoretical Chemistry, University of Bonn)

Photostability of DNA

Electronically excited states of DNA building blocks are thought to be short-lived in order to minimize the potential for photoinduced genetic damage resulting from photochemical reactions such as excited state proton transfer. This ab initio simulation investigates possible light-induced solvent-assisted proton transfer mechanisms leading to tautomerization of the DNA base guanine in aqueous solution. The snapshot illustrates that the highest singly occupied molecular orbital (yellow and green contours) of the first excited singlet state becomes delocalized over the solvent molecules upon transferring a proton from guanine to the solvent, whereas the lowest singly occupied molecular orbital (blue and brown contours) remains localized on guanine.

(Nikos Doltsinis, Theoretical Chemistry, University of Bochum)

Electronic Conduction on the Molecular Scale

The extrapolation of present trends in the packing density of electronic components may require the construction of single-molecule or single-atom electronic components in the foreseeable future. Recent experimental advances make it possible to investigate the electronic conduction properties of individual molecules, but a detailed theoretical understanding is still lacking. We have developed quantitative models for the description of the electronic conduction properties of individual molecules by combining detailed electronic structure theory with models for electronic transport in mesoscopic systems. On the basis of these models we were able to explain the current voltage characteristics of several molecules that were experimentally investigated and to predict novel effects that are presently being studied.

(Wolfgang Wenzel, Institute for Nanotechnology, Forschungszentrum Karlsruhe)

Transition Metal Ions in Biological Matrices

The incorporation of metal ions in biological matrices results in the formation of metalloenzymes, which in the case of transition metal ions can lead to redox active catalysts. Vanadium haloperoxidases are a group of enzymes that take part in the biosynthesis of halogenated natural products. The picture shows the structure of the vanadium haloperoxidase from the fungus Curvularia inaequalis. The chemical analogy between vanadate and phosphate also has consequences for biological systems. In particular, it is known that the active site structure of a group of acid phosphatases is similar to that of the apo-protein of the vanadium haloperoxidases. How does nature utilize one given biological matrix to perform two very different chemical reactions (i.e. oxidation in vanadium haloperoxidases and hydrolysis in phosphatases)? This is related to structural and mechanistic questions which are the subject of the current investigations. The insets show the frontier orbitals of a vanadate imidazole moiety found in the active site of vanadium haloperoxidases (right: HOMO, left: LUMO).

(Masroor Bangesh, Axel Pohlmann, Winfried Plass, Inorganic Chemistry, Friedrich-Schiller-University Jena )

Intro- duction	Scientific Computing	Astro- physics	Elementary Particles	Multi- particles	Polymers	Chemistry	Earth, En- vironment	Other Fields