IFF
Scientific Report 1999/2000

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.

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 spans the entire physics of condensed materials! The institute’s 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 and Dr. Ballone. A new 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). A perennial challenge to the condensed matter theorist is the problem of interactions and correlations among electrons. Whereas for s- and p-electron systems these effects are satisfactorily taken into account by density functionals, d- and f-electron systems show phenomena which are beyond the scope of LDA. Methods like dynamical mean field theory, quantum Monte Carlo calculations and exact diagonalization will be applied for those systems by Dr. Liebsch. Work in this direction is reported below.

Dr. Harris returned to the institute after a five years leave of absence during which he worked in a US company providing scientific software. He builds on the experience of many years by applying density functional theory to materials problems, and together with Dr. Bringer is establishing a programme to apply data mining methods for materials research at the institute, as described below.

An important research topic in the Institute is the atomic theory of friction and the related issue of crack propagation. A new result is presented below as an example of the general area of research. Over the past years, Dr. Persson had 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 and applications.

Another research area of the Institute, the theory of non-linear systems, had been transferred for the period 1997-99 to the ‘Modelling Forum', where Dr Lustfeld, co-opted as a member of the Forum, was applying his know-how to environmental problems. This multi-institutional approach to interdisciplinary tasks will retain support from theory, in line with the research policy of our research center.

The research area of Prof. Eisenriegler - ‘Geometric Effects in Complex Fluids' - bears a close relationship to the theory of soft-matter. At the end of this year, Prof. Eisenriegler will join the Institute Theory II and take this activity along with him. 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.

• 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. An extension of this funding for the years 2000-2002 is pending.

Gert Eilenberger