To study chemical reactivity and spectroscopic properties of complex systems like proteins or organic semi-conductors, quantum mechanical (QM) methods are required. Due to the large size of these structures, a simple application of QM methods is impossible due to the high computational cost. Therefore, QM methods are conveniently combined with empirical force field methods (Molecular Mechanics: MM) in the so- called QM/MM methods. In such approaches, the active site of a protein consisting of several tens of atoms is treated with QM, while the remainder of the protein is handled with the computationally cheaper MM methods. This approach can be taken to treat even larger scales by combining the QM/MM methods with continuum approaches. Also, various QM methods with different accuracy can be applied in the active site, building onion-like computation shells of methods.
Besides the development of such method-combinations, a large part of our effort concentrates on the development of fast and accurate approximations to Density Functional Theory (DFT) resulting in the DFTB suite of methods which are about three orders of magnitude faster than DFT, however, keeping a similar accuracy for a broad range of molecular properties.