A tribute to Thomas Frauenheim

Abstract

This special issue of PSS is in honor of Thomas Frauenheims 60th birthday. His central theme always was the understanding of what he called real materials properties, meaning systems of a size which makes a comparison of their computed properties with experimental observables meaningful. In the 1990's he started to use a DFT based tight-binding (TB) method, and he very quickly became a driving force in developing the applications and capabilities of this methodology. While being originally an approximation to DFT-LDA, Thomas has realized its formal similarity to tight-binding methods used in solid state physics. These TB methods were known to be quite accurate for the materials classes they have been parameterized for, but the parameterization itself could be quite tedious and time-consuming. The approximate DFT method could be viewed as a simple way of getting TB parameters, therefore Thomas introduced the name Density-Functional Tight-Binding (DFTB) for this methodology. Starting out with studies on complex carbon systems, studying their structure and spectroscopic properties, he quickly became interested in a broad range of materials, from molecules, clusters, surface problems, over voids and vacancies in semi-conductor materials to biological molecules. The atomistic description of nano-scale materials was and is in the center of his interest. DFTB has been the workhorse in all these years, although he has never been limited to a single methodology. His science was always rather problem than methodology driven, therefore he augmented the semi-empirical DFTB calculations at both ends with other methods, by empirical force fields on the one hand or plane wave DFT and even post-Hartree Fock methods, on the other, when very accurate calculations have been required. And of course, the nature of those systems often asked for so called ‘multi-scale’ solutions, i.e. a combination of various methods for the different scales in one methodology, like in the QM/MM approaches. Beside that, however, much of his efforts was directed into the extension of DFTB's capabilities in order to cover various materials properties, as there are, for example, vibrational spectra of molecules and solids, optical properties of molecules and clusters within a DFTB adapted time-dependent DFT (TD-DFT) approach, non-adiabatic molecular dynamics simulations to model pump-probe processes, calculations of hyperfine coupling constants for radicals and magnetic properties of clusters with a spin-polarized extension of DFTB, calculations of scanning probe (STM) images of surfaces and the calculation of electronic transport properties within the non-equilibrium Green's function technique, to mention only some of the applications and methodological extensions of the DFTB. All this of course was not possible with the capabilities of a single work group. One of Thomas' most outstanding qualities is his openness and ability to bring co-operations into a family-like setting. Over the years, he has been closely working with various groups in several countries on different continents. These co-operations contributed significantly to the expertise, which is now assembled in the computational machinery, partly manifested in the DFTB+ program suite, partly in many other pieces of code being available now for DFTB. His attitude was never one of competition but rather of cooperation. We hope that he will continue in this spirit to further inspire materials science research in the future. Gotthard Seifert, Marcus Elstner, and Peter Deák (Guest Editors)