IFF
Scientific Report 1998/1999

The main focus in the Institute is on gaining an understanding of electronic structure and atomic-scale processes and, wherever realistically possible, performing quantitative calculations. The term 'quantitative' is used here to draw a distinction to the other important 'qualitative' aspect of theory, which considers characteristic elements of physical reality in the abstract and represents these using the simplest possible theoretical or numerical representation. The individual themes that constitute this area of activity will be described in more detail below.

Another research area of the Institute, the theory of non-linear systems, has been transferred to the 'Modelling Forum', where Dr Lustfeld, co-opted as a member of the Forum for the the period 1997-99, is applying his know-how to environmental problems. This, together with activities of the Forum Members Dr. Baumgärtner (Theory II) and Dr. Zimmerman (Theory III, now the University of Saarbrücken) and doctoral students of the Forum, constitutes a purely theoretical, multi-institutional approach to interdisciplinary tasks. This activity will retain its support from theory in the future.

The research area of Prof. Eisenriegler - 'Geometric Effects in Complex Fluids' - bears a close relationship to the theory of soft-matter, though it remains a separate activity. The work studies the forces that operate between geometric objects - walls, spheres (colloids) and cylinders having dimensions in the mesoscopic range - which arise as a result of thermodynamic effects in the surrounding fluid. Examples are critical fluid mixtures and dilute polymer solutions. The methods employed in these studies include analytical, field-theoretical approaches as well as numerical simulations in collaboration with Dr. Bringer, who contributed methodologies and programmes. In addition, Dr. Bringer has established a close collaboration with the experimental Institute of Prof. Lengeler, RWTH Aachen, with regard to the quantum-mechanical treatment of interference effects in semiconductor-nanostructures.

For a given collection of nuclei, the structure and properties of the resulting atomic assemblies are determined in the last analysis by the electrons. Thus the main focus of the Institute's work programme, referred to above, in fact includes the entire physics of condensed materials! A choice of relevant problems in this area, which is addressed by a virtual army of theorists worldwide, is made according to the following aspects:

• current issues of a methodological or thematic nature,
• the expertise of staff members,
• the availability of computer programme-packages that have evolved over many years (and so can be compared to moderately expensive experimental equipment),
• special capabilities of the Jülich computer facility,
• status of competition and possibilities for collaboration, local, national and international.

In this sense, a central feature is the ongoing development and application of current density-functional/ab initio molecular dynamics programmes, especially in conjunction with the powerful computational facility in ZAM. Work in this area has been performed by Dr. Jones, Dr. Lichtenstein, Dr. Ballone, Dr. Kaschner and Ms Montanari (student). An example of work in this research area can be found below.

The computer programmes run currently on the T3E. On-going development includes the testing of new, non-local approximations for the exchange-correlation energy functional. Such approximations are essentially empirical and not the result of ab initio theory. This leads to systematic deviations with respect to measured values and with values calculated for small systems by more reliable, but much more resource intensive quantum methods. Combining accuracy with ease of computation will remain a challenge into the future. The focus of applications has been to atomic clusters, especially those with light atoms (organic molecules, carbon-aggregates, systems containing water), which require ‘hard' pseudopotentials and so represent difficult numerical problems. Other applications include energy surfaces and phonons in glasses.

A further area of research concerns the interaction of electrons and electromagnetic fields in bulk materials (Prof. Sturm) and on sufaces (Dr. Liebsch). In the former case, new results constitute a consistent calculation of the dielectric function e (k, w). In the latter, the focus is on the calculation of linear and non-linear optical response on clean and adsorbate covered surfaces, an area of especially active research for spectroscopy and diagnostics at surfaces. An example of work in this area can be found below.

An important research topic in the Institute is the atomic theory of friction and the related issue of crack propagation in polymers. This work is summarized below as an example of the general area of research. Over the past four years, Dr. Persson has pioneered work in this area (much of which is summarized in his 1997 monograph on this topic). This remains an area whose development is just breaking out of infancy, and has broad-based ramifications and many open questions.

• Dr. Liebsch was awarded a grant from the EU to conduct a collaboration with South-American Universities over the period 1996-98. The grant covered Dr. Liebsch's travel and local expenses.
• Dr. Hohl received financial support in the form of a BAT IIa position from the DFG (German Research Association) to assist pursuit of a research programme on ‘Computersimulations in Materials Research'. This was used to finance the position of Dr. Kaschner up to May 1998.
• Between 1994 and 1999, Dr. Persson was granted support by the DFG to cover annual 3-month guest visits to IFF by Prof. Volokitin, University of Samara, Russia.
• As a participant on a 5-year BMBF(German Research Ministry)-supported German-Israeli Cooperative Project, Dr. Persson receives from 1998 annual funding of 50,000 DM to cover guest visits and travel expenses.
• As a contributor in a collaboration between institutes from Jülich, Mainz and Heidelberg, Dr. Jones received for the years 1996-98 from the MaTech Programme of the BMBF funding of in total 1,141,000DM. These funds were used in part to finance the positions of Dr. Lichtenstein and Ms Montanari. An extension of this funding will be applied for as soon as the next round of applications is announced.
• Within the framework of the German-Chinese Exchange Programme, funding for a month's visit to the Zhejiang University, Hangzhou was made available to Prof. Eilenberger.
• Under the auspices of the German-Hungarian Exchange Programme, Dr. Lustfeld was awarded funding during the period 1996-98 for Hungarian guests. The funding covered visits up to 100 days annually. Unfortunately, this funding was not extended.

• The behavior of mesoscopic particles dissolved in a dilute solution of long, flexible, and nonadsorbing polymer chains is studied by field-theoretic methods. For spherical and cylindrical particles the solvation free energy for immersing a single particle in the solution is calculated explicitly. For the effective interactions between a small sphere and a wall, between a thin rod and a wall, and between two small spheres quantitative results are presented. A systematic approach for studying effective many-body interactions is provided.

• The MD/MF method in conjunction with all-electron DF calculations were applied succesfully to a series of molecules. Examples are: a) Calculations for bisphenol-A-polycarbnate-oligomer crystals. The agreement with the measured structure is very good. b) Structural trends and trends in the vibration frequencies of polycarbonate molecules and their fragments led to a detailed picture of the energy surface of the polycarbonates. c) Cluster calculations showed unexpected geometrie - as is usually the case. Work on carbon clusters with 4 - 32 atoms showed structures that were previously not anticipated, and gave a satisfying picture of structural changes with increasing cluster-size.

• Optimized structures and energies for several isomers of glyzin and alanin were calculated systematically using the LDA and three different gradient corrections. The results were compared with those obtained via considerably more expensive Hartree-Fock calculations. The agreement was generally good except for the isomers where intra-molecular hydrogen bonds occur. The use of gradient corrections improved the agreement considerably leaving only small deviations (of approximately 40meV in the relative energies). Correspondingly, gradient corrections are necessary for a better description of hydrogen bonds. In addition, the energy changes accompanying torsional changes around bond directions could be determined.