My main research theme is the development and application of quantum chemical methods used in a multiscale-modeling approach of molecular systems. Aspects of this work are the definition of subsystem approaches, the connection between wave function and density functional theory, and the exploration of quantum computers. In the electronic structure methods particular attention is paid to the simultaneous treatment of electron correlation and relativity. Aim is a precise treatment of large molecular systems with an economical and comprehensible interpretation of the influence of the environment on local molecular properties. Applications are found in various areas of chemistry and physics, recent work concerns e.g. photochemistry of biomolecules, actinide chemistry, benchmark calculations of molecular properties.